How to Measure a Stamp: Character Size and the Center to Center Method
Short answer. Measure a metal stamp center to center. Put your reference on the middle of the top stroke of the character and the middle of the bottom stroke. That dimension is the character size, and character size is the number stamps are sold by.
Do not measure the outside of the impression, top edge to bottom edge. A stamped impression is always larger than the stamp that made it, so an outside reading sends you one size too big.
This is the reference page we send customers who are about to order a custom stamp and are not sure what size to ask for. It covers the measurement itself, why the two numbers on a stamped part are different, what to measure with, the full size chart in fractions, decimal inches and millimeters, and how the answer shifts by material and by project.
It applies to steel hand stamps, punches, and dies of every kind: restoration and VIN work, maker's marks and touchmarks, jewelry hallmarks, leather tooling, data plates, and industrial part marking. It does not cover postage stamps or rubber office stamps.
The four dimensions on a stamp
Most sizing mistakes are vocabulary problems, not measuring problems. A single hand stamp has four different dimensions and only one of them is the size you order.
- Character size
- The height of the letter, number, or logo on the face of the tool, measured center to center of the strokes. This is the number stamps are sold by and the number you order. Also called character height or cap height.
- Stamped impression size
- The full height of the mark left in the part, outer edge to outer edge. Always larger than the character size, and it changes with how hard the part was struck.
- Face size
- The overall footprint of the working end of the tool. On a logo stamp this is the outline of the whole mark. It matters for clearance on a small part, not for character sizing.
- Shank size
- The square or round body of the tool you hold or chuck. On a 1/8 inch character stamp the shank is often 5/16 or 3/8 inch. People occasionally measure this and report it as the stamp size. It is not.
When a supplier says 1/8 inch stamp, they mean 1/8 inch character size. Every reputable maker in the trade sizes tools this way, so once you have the character size you can order from anyone and get the same letter height.
The center to center rule
Character size is measured from the very top center point of the character to the very bottom center point. On a capital E that is the middle of the top bar to the middle of the bottom bar. On a round character like an 8 or an O it is the middle of the stroke at the top of the curve to the middle of the stroke at the bottom.
This is the standard across the hand stamp trade, and there is a good engineering reason for it. The centerline of a stroke is the only feature on a stamped mark that does not move. Outer edges move with strike force, material hardness, and how much metal flowed. Stroke centers stay put. Measure the thing that repeats.
The rule in one line. Center of the top stroke to center of the bottom stroke. Not edge to edge, not including the raised lip, not the tool body.
Why the impression is bigger than the stamp
A hand stamp does not have vertical walls. The character stands proud of the face with tapered flanks running down to the base. That taper is what keeps the letter from shearing off after a few thousand strikes.
When the stamp is driven in, the character sinks and the displaced material has to go somewhere. It flows outward and up the taper, and it piles into a small raised lip around the mark. The harder the strike and the softer the material, the further that lip spreads.
So the outer boundary of an impression is not the edge of the character. It is the edge of the displaced material. Two identical stamps, struck by two different people on the same part, produce two different outside measurements and one identical center to center measurement. That is the whole argument for center to center in a sentence.
Practical size of the effect: on mild steel, an outside reading typically runs 10 to 20 percent over the true character size. On aluminum or soft brass, struck hard, it can run further. Order off that number and you get a stamp a full size bracket too big.
How to measure an existing stamped impression
This is the case that matters most in restoration work: there is a mark on the part and you need the stamp that made it.
- Clean the surface. Degrease it and get paint, filler, and rust out of the groove. Anything sitting in the impression rounds the edges and hides the stroke. A brass brush and solvent is usually enough. Do not media blast an original stamping you intend to match, because blasting erodes the edges you are trying to read.
- Find the tallest full-height character in the line. Capital letters and numerals share a height. Skip lowercase letters, commas, periods, and symbols.
- Identify the top and bottom strokes. On a capital E, the top bar and the bottom bar. On an 8, the top of the upper loop and the bottom of the lower loop. On a 1, the top of the flag and the foot.
- Measure center to center. Land your reference on the middle of the top stroke and the middle of the bottom stroke. Ignore the outer edges. Ignore the raised lip.
- Repeat on two or three more characters. Old stampings are rarely struck square. Take three readings and use the median, not the largest.
- Round to the nearest standard size. Stamps are made in set increments. If you read 0.121 inch you are looking at a 1/8 inch stamp, not a custom size. See rounding below.
How to measure a stamp you already own
Measuring the tool directly is harder than it looks, because the character is small, raised, and tapered, and calipers want to ride up the flank. The trade method is to make a test impression and measure that instead.
- Lay a sheet of plain paper on a hard flat surface. A steel plate or a granite block. Not a wooden bench top, which gives and shallows the mark.
- Press or tap the stamp into the paper. You want a clean embossed outline, not a hole. Firm hand pressure is often enough on paper. A single light tap works too.
- Measure the paper impression center to center. Paper barely flows, so the impression sits very close to true character size. That is exactly why this method works.
- Cross-check on scrap. If you have a scrap of the material you actually intend to mark, strike that too. Now you have both numbers: the character size you order by, and the impression size that part will show.
If the mark is a logo rather than text, measure the overall height of the logo the same way, top center to bottom center of the outermost strokes, and give us the width as well. Logos are sized by their bounding box, not by cap height.
Measured it and still want a second opinion before you spend money? Send it in on a quote request with a straight-down photo and we will confirm the size for you at no charge.
What to measure with
In rough order of how well each one works on a real part:
- Jeweler's loupe and a graduated steel scale. A 10x loupe with a 0.5 mm scale laid across the character is the most reliable low-tech method for small marks. You can see exactly where the stroke centers sit, which is the whole game.
- Photograph with a reference in frame. Lay a steel rule or a coin flat against the part, in the same plane as the stamping, shoot straight down, and measure on screen. This is what we ask for most often, because we can verify the scale ourselves instead of trusting a typed number.
- Digital calipers, knife edges or depth blade. Fine for characters around 3/16 inch and up. Below that it is hard to land the jaws on stroke centers rather than edges.
- Optical comparator or measuring microscope. The correct tool if you have access to one. Most people do not, and most people do not need one.
Shoot straight down, not at an angle. A photo taken 30 degrees off perpendicular can read several percent short, which is enough to push you into the wrong size bracket. If the stamping is on a curved surface, shoot perpendicular to the tangent at the mark, not perpendicular to the part.
Stamp size chart, fractions to millimeters
Stamps are sold by character height. American work is normally specified in fractions of an inch, European and Japanese work in millimeters. Use this to turn what you measured into a size you can order.
| Fraction | Decimal inch | Millimeters | Typical use |
|---|---|---|---|
| 1/32" | 0.031 | 0.79 | Hallmarks, ring shanks, watch parts |
| 1/16" | 0.063 | 1.59 | Jewelry marks, small data plates |
| 5/64" | 0.078 | 1.98 | Fine serial work |
| 3/32" | 0.094 | 2.38 | Precision parts, medical instruments |
| 1/8" | 0.125 | 3.18 | The most common size. Engine codes, date codes, jewelry, fine leather |
| 5/32" | 0.156 | 3.97 | Data tags, tooling identification |
| 3/16" | 0.188 | 4.76 | VIN and frame stamps, medium leather, small knives |
| 7/32" | 0.219 | 5.56 | Less common, occasional factory sizes |
| 1/4" | 0.250 | 6.35 | Chassis and body stamps, touchmarks, general fabrication |
| 5/16" | 0.313 | 7.94 | Heavier fabrication, larger touchmarks |
| 3/8" | 0.375 | 9.53 | Blacksmith touchmarks, structural marking |
| 1/2" | 0.500 | 12.70 | Structural steel, pipe, pressure vessels |
| 5/8" | 0.625 | 15.88 | Heavy industrial, forged billet marking |
| 3/4" | 0.750 | 19.05 | Press work, large signage dies |
| 1" | 1.000 | 25.40 | Press only. Too large to drive by hand in steel |
| You measured | Decimal inch | Nearest standard fraction |
|---|---|---|
| 1.0 mm | 0.039 | 1/32" |
| 1.5 mm | 0.059 | 1/16" |
| 2.0 mm | 0.079 | 5/64" |
| 2.5 mm | 0.098 | 3/32" |
| 3.0 mm | 0.118 | 1/8" |
| 4.0 mm | 0.157 | 5/32" |
| 5.0 mm | 0.197 | 3/16" |
| 6.0 mm | 0.236 | 1/4" |
| 8.0 mm | 0.315 | 5/16" |
| 10.0 mm | 0.394 | 3/8" |
| 12.0 mm | 0.472 | 1/2" |
If you need an exact metric character height rather than the nearest fraction, say so on the quote request. We cut custom sizes. Most people are better served by a standard size, because it matches the tooling already on the part.
Have us confirm your size and quote the stamp
Rounding to a standard size
Your reading will not land exactly on a chart value, and it should not. Wear, corrosion, and the flow of the material all move the number a little. Here is how to resolve it.
- Within about 8 percent of a standard size, round to that size. A reading of 0.118 to 0.135 inch is a 1/8 inch stamp.
- Landing squarely between two sizes usually means you measured edge to edge. Go back and re-read center to center before you conclude you need an odd size.
- On a worn original, round down, not up. Wear opens a mark up. It does not shrink it.
- Factory stampings are almost always standard sizes. If your number says otherwise, the measurement is more likely wrong than the factory.
How many characters fit in your space
The second question after height is always width. You have a boss, a tag, or a blade flat of a known length and you need to know whether your text fits.
For the gothic sans faces used on hand stamps, plan on roughly 0.75 times the character height per character, including the gap between characters. That gives a working rule:
Line length = number of characters × character height × 0.75
| Character height | Approx. characters per inch | Length of a 12 character line |
|---|---|---|
| 1/16" | 21 | 0.56" |
| 3/32" | 14 | 0.84" |
| 1/8" | 11 | 1.13" |
| 3/16" | 7 | 1.69" |
| 1/4" | 5 | 2.25" |
Spacing, centering, and keeping the line straight are their own subject. Full method in how to lay out stamped lettering.
Two adjustments. Hand stamping one character at a time is under your control, so you can tighten or open the spacing to fit. A one-piece die or a holder set is fixed at the layout we cut, so tell us the maximum width you have and we will size the text to it. And if you are matching an original factory line, measure the original line length as well as the character height. Factory spacing was often tighter than a hand stamper would naturally set it.
Working out whether your text fits is the fastest thing for us to check. Send the available width and the text and we will size it.
How material changes the read
Character size never changes. What changes is how far the impression spreads past it, how deep you can drive, and how much of the mark you can actually see when you go to measure it later.
| Material | Impression spread | Notes when measuring |
|---|---|---|
| Mild and low carbon steel | Moderate | The reference case. Clean edges, predictable lip, easy to read center to center |
| Stainless steel | Low | Work hardens, so depth varies across a line. Read a mid-line character, not the first one |
| Hardened or heat treated steel | Very low | Shallow and faint. Use raking light at a low angle to find the stroke centers |
| Cast iron | Low, ragged | Grain tears at the edges. Centers are still reliable, edges are useless |
| Aluminum | High | Flows hard and throws a big lip. Edge to edge readings here are the most misleading of all |
| Brass and bronze | Moderate to high | Clean and crisp. Polishing after stamping rounds the edges, so measure before you polish |
| Copper | High | Very soft. Easy to over-drive and distort the character shape |
| Sterling silver and gold | High | Small marks on soft metal. A loupe is not optional here |
| Vegetable tanned leather | Variable with moisture | Cased leather spreads more than dry. Measure the tool on paper, not the mark in the hide |
| Plastics and acrylic | Low, can crack | Cold stamping risks stress cracks. Hot stamping is the better route |
| Wood | Grain dependent | Late wood resists, early wood crushes. Branding gives a truer size than cold stamping |
The takeaway holds across all of them. Order by character size, and expect the mark on the part to look bigger than the number you ordered.
Measuring by project type
Restoration, VIN, and factory stampings
The goal is to match an original, so accuracy matters more here than anywhere else. Measure at least three characters and use the median. Photograph the original line with a scale in frame and send it with your quote, because the font matters as much as the size and we can read the font off a good photo. Note whether the line mixes sizes. Factory data plates frequently run a smaller prefix or suffix beside a larger serial, and matching one size to the whole line is the most common restoration mistake we see. Our marque guides for Ford Bronco, GM engine codes, and BMW list known sizes by year and location.
Data plates, tags, and industrial part marking
Legibility at reading distance sets the size, and the plate sets the ceiling. Work backward: take the usable width of the plate, divide by the longest line of text, and use the line length formula above to find the largest height that fits. If the mark has to survive paint, plating, or powder coat, go one size up and drive it deeper. See our guide to custom data tags and data plates.
Maker's marks and touchmarks
Knife makers usually land between 1/4 and 3/8 inch overall mark height. Blacksmiths working larger stock go 3/8 to 5/8 inch. Detail is the limit, not the tool. A logo with fine internal lines will not hold together below about 1/4 inch in steel, so simplify the artwork or go bigger. Full detail in our touchmark guide.
Jewelry and hallmarks
Small work, 1/32 to 1/16 inch for hallmarks and quality marks, up to 1/8 inch for a name or logo on a pendant or cuff. Soft metals flow, so the finished mark reads noticeably larger than the character size. Curved surfaces such as ring shanks need a curved or narrow face so the whole character lands at once.
Leather
Leather takes a bigger mark than metal for the same visual weight, because the impression edges are soft. 1/8 inch for fine goods and wallet interiors, 3/16 to 1/4 inch for belts, straps, and bag panels. Measure the tool on paper rather than the mark in the hide, since cased leather spreads and dry leather does not. Technique notes in our leather, blades, and jewelry guide.
Mistakes that cost a re-order
- Measuring outer edge to outer edge. The big one. It reads oversize and the new stamp will not match the original stamping.
- Measuring the raised lip. Displaced material around the mark is not part of the character.
- Measuring a lowercase letter or a punctuation mark. Size is set by cap height and numeral height.
- Measuring a worn or double struck character. A character hit twice at slightly different positions reads tall. Find a clean one.
- Assuming the whole line is one size. Original stampings frequently mix sizes. Measure each group separately.
- Measuring the tool body instead of the character. The shank is not the stamp size.
What to send us for a quote
Send these five things and we can size the tool without a round of questions.
- The character size you measured, center to center, with the unit.
- A straight-down photo of the original mark with a rule or a coin in the same plane.
- The exact text or the logo artwork, ideally as vector.
- The material you are marking and its rough hardness or temper.
- The space you have, both the width available and any curvature.
Not sure what you need yet? Browse custom stamps or send the photo and we will tell you the size.
Frequently asked questions
How do you measure a stamp?
Measure the character center to center, from the middle of the top stroke of the character to the middle of the bottom stroke. That dimension is the character size, which is the size stamps are sold and ordered by.
Do you measure the outside of the impression or the center of the lines?
The center of the lines. Outer edges shift with strike force and material, so an edge to edge reading is not repeatable and typically runs 10 to 20 percent oversize.
Why is my stamped impression bigger than the stamp that made it?
The character has tapered flanks, and struck material flows outward and up the taper into a raised lip around the mark. The outer boundary of the impression is the edge of that displaced material, not the edge of the character.
What size stamp do I need?
Match an existing mark by measuring it center to center. Starting fresh, 1/8 inch suits jewelry, fine leather, and small parts, 3/16 to 1/4 inch suits VIN and chassis work, blades, and belts, and 3/8 inch and up suits blacksmith touchmarks and structural marking.
Is stamp size the letter height or the size of the tool?
The letter height. A 1/8 inch stamp has a 1/8 inch character on a shank that is often 5/16 or 3/8 inch square. The shank is not the size.
How do I measure a stamp I already own?
Press it into paper on a hard flat surface and measure the paper impression center to center. Paper barely flows, so the impression sits very close to true character size.
How do I measure the numbers on a VIN plate or data tag?
Clean the plate, pick the tallest clean numeral, and measure center to center on three characters. Use the median. Note whether prefix or suffix characters are a different size from the serial, because factory plates often mix sizes in one line.
What if the impression is worn, painted over, or partly filled?
Degrease and clean the groove with solvent and a brass brush, then read it under raking light at a low angle so the stroke shadows stand out. Do not media blast an original you intend to match. If several characters are gone, measure a clean one elsewhere on the part and round down rather than up.
Should I order in inches or millimeters?
Use whichever unit you measured in and tell us which it is. American tooling is usually specified in fractions of an inch, European and Japanese tooling in millimeters. The conversion tables above cover both directions.
Does the material change the size I should order?
No. Character size is a property of the tool, so you order the same size regardless of material. What changes is how far the mark spreads past that size and how deep you can drive it. Soft metals such as aluminum and copper spread the most.
How many characters will fit in the space I have?
Multiply the number of characters by the character height, then by 0.75. Twelve characters at 1/8 inch run about 1.13 inches. Send us the available width and we will size the text to fit.
Can you match an original factory font as well as the size?
Yes. Send a straight-down photo of the original mark with a scale in frame. We read the font from the photo and cut to match. Size alone rarely makes a restoration stamping look right.
Related guides
- How to lay out stamped lettering: straight lines, even spacing, centered text
- How to make a touchmark
- Using metal stamps on leather, blades, and jewelry
- Custom data tags and data plates
- Custom stamps
- Request a quote
RestoStamps has been cutting custom steel stamps for restoration shops, makers, and manufacturers since 2010. Questions on sizing go to Sales@RestoStamps.com.