Nail Size Converter
Nail Size Converter
Penny sizes (2d, 6d, 16d) to length in inches and millimetres, shank diameter and wire gauge — with the two facts every chart hides: the (d + 2) ÷ 4 rule fails above 10d, and a 16d common, box and sinker are the same length and three different nails.
Nail sizes
a 16d common nail in Douglas-fir
A penny is a length; a type is a diameter
- d
- penny, from the price in pence of a hundred nails in 15th-century England. It designates length and nothing else
- (d + 2) ÷ 4
- exact from 2d to 10d, wrong from 12d up, and wildly wrong at the top: it gives 15½ in for a 6-inch 60d
- common / box / sinker
- same penny, same length, three different shank diameters. A 16d box is 0.135 in against a common’s 0.162
- K
- Wood Handbook table 8-4, by species specific gravity: 1,440 to 2,720 in pounds and inches
- D3/2
- why a 17% thinner shank loses 31% of its capacity. The exponent is the whole reason type matters
Worked example
a 16d common nail in Douglas-fir
16d is 3½ inches long by the published table: 3.5 × 25.4 = 88.9 mm
The famous rule would give (16 + 2) ÷ 4 = 4½ in, a full inch too long. It has not worked since 10d
The shank is 0.162 in — 4.115 mm — on the code family. The wire-gauge chart says 0.165 for the same nail
A 16d BOX is the same 3½ inches long and only 0.135 in thick, and a 16d sinker is 3¼ by 0.148. Three nails, one label
Wood Handbook equation 8-2 with K = 2,200: p = 2,200 × 0.162^1.5 = 143.4 lb (638 N)
The same estimate for the 16d box gives 109.1 lb, so the common is 31.5% stronger — inside the 22–44% a trade source publishes from rated capacities
In metric terms this nail is a 88.9 × 4.11 mm; the nearest stocked metric length is 90 mm
Where the famous rule stops working
| Penny | Published length (in) | … (mm) | (d + 2) ÷ 4 (in) | Error (in) | Verdict |
|---|---|---|---|---|---|
| 2d | 1.00 | 25.4 | 1.00 | 0.00 | exact |
| 3d | 1.25 | 31.8 | 1.25 | 0.00 | exact |
| 4d | 1.50 | 38.1 | 1.50 | 0.00 | exact |
| 5d | 1.75 | 44.4 | 1.75 | 0.00 | exact |
| 6d | 2.00 | 50.8 | 2.00 | 0.00 | exact |
| 7d | 2.25 | 57.1 | 2.25 | 0.00 | exact |
| 8d | 2.50 | 63.5 | 2.50 | 0.00 | exact |
| 10d | 3.00 | 76.2 | 3.00 | 0.00 | exact |
| 12d | 3.25 | 82.5 | 3.50 | 0.25 | wrong |
| 16d | 3.50 | 88.9 | 4.50 | 1.00 | wrong |
| 20d | 4.00 | 101.6 | 5.50 | 1.50 | wrong |
| 30d | 4.50 | 114.3 | 8.00 | 3.50 | wrong |
| 40d | 5.00 | 127.0 | 10.50 | 5.50 | wrong |
| 60d | 6.00 | 152.4 | 15.50 | 9.50 | wrong |
Shank diameters: same length, different nails, and two families that disagree
| Penny | Length (in) | Common, code family | Box, code family | Sinker, code family | Common, wire-gauge chart | Box, wire-gauge chart | Wire gauge (common) |
|---|---|---|---|---|---|---|---|
| 2d | 1.00 | — | — | — | 0.072 | — | 15.00 |
| 3d | 1.25 | — | — | — | 0.083 | 0.076 | 14.00 |
| 4d | 1.50 | — | — | — | 0.109 | 0.080 | 12.50 |
| 5d | 1.75 | — | — | — | 0.109 | 0.080 | 12.50 |
| 6d | 2.00 | 0.113 | — | — | 0.120 | 0.098 | 11.50 |
| 7d | 2.25 | — | — | — | — | 0.098 | — |
| 8d | 2.50 | 0.131 | 0.113 | — | 0.134 | 0.113 | 10.25 |
| 10d | 3.00 | 0.148 | — | — | 0.148 | 0.128 | 9.00 |
| 12d | 3.25 | 0.148 | — | — | 0.148 | — | 9.00 |
| 16d | 3.50 | 0.162 | 0.135 | 0.148 | 0.165 | 0.135 | 8.00 |
| 20d | 4.00 | 0.192 | — | — | 0.203 | 0.148 | 6.00 |
| 30d | 4.50 | — | — | — | 0.220 | — | 5.00 |
| 40d | 5.00 | — | — | — | 0.238 | — | 4.00 |
| 60d | 6.00 | — | — | — | 0.238 | — | 2.00 |
Why substituting a box nail is not free — Douglas-fir, K = 2,200
| Penny | Common Ø (in) | Common p (lb) | Box Ø (in) | Box p (lb) | Common stronger by | Sinker Ø (in) | Sinker p (lb) | Common stronger by |
|---|---|---|---|---|---|---|---|---|
| 6d | 0.113 | 83.6 | — | — | — | — | — | — |
| 8d | 0.131 | 104.3 | 0.113 | 83.6 | 24.8% | — | — | — |
| 10d | 0.148 | 125.3 | — | — | — | — | — | — |
| 12d | 0.148 | 125.3 | — | — | — | — | — | — |
| 16d | 0.162 | 143.4 | 0.135 | 109.1 | 31.5% | 0.148 | 125.3 | 14.5% |
| 20d | 0.192 | 185.1 | — | — | — | — | — | — |
The Wood Handbook’s K, both unit systems, checked against each other
| K (lb, in) | K (N, mm) as published | Species group | K (N, mm) computed from the lb column |
|---|---|---|---|
| 1440 | 50.04 | softwood, specific gravity 0.29–0.42 (spruce, fir, cedar) | 50.04 |
| 1800 | 62.55 | softwood, specific gravity 0.43–0.47 (hem-fir, redwood) | 62.55 |
| 2200 | 76.45 | softwood, specific gravity 0.48–0.52 (Douglas-fir, southern pine) | 76.45 |
| 1440 | 50.04 | hardwood, specific gravity 0.33–0.47 (aspen, basswood) | 50.04 |
| 2000 | 69.50 | hardwood, specific gravity 0.48–0.56 (birch, soft maple) | 69.50 |
| 2720 | 94.52 | hardwood, specific gravity 0.57–0.74 (oak, hickory, hard maple) | 94.52 |
Out of the penny system: metric nails are length × diameter
| Penny and type | Length × diameter (in) | Length × diameter (mm) | Nearest stocked metric length (mm) |
|---|---|---|---|
| 6d common | 2 × 0.113 | 50.8 × 2.87 | 50 |
| 8d common | 2½ × 0.131 | 63.5 × 3.33 | 65 |
| 10d common | 3 × 0.148 | 76.2 × 3.76 | 75 |
| 16d common | 3½ × 0.162 | 88.9 × 4.11 | 90 |
| 16d box | 3½ × 0.135 | 88.9 × 3.43 | 90 |
| 16d sinker | 3¼ × 0.148 | 82.5 × 3.76 | 75 |
| 20d common | 4 × 0.192 | 101.6 × 4.88 | 100 |
The rule fails above 10d, and the type sets the diameter
A penny size is a length and nothing else. The ‘d’ is a denarius: the price in pence of a hundred nails of that size in fifteenth-century England, which is a price list that outlived its currency. Nothing about it is systematic, and that is why the rule everybody quotes — length in inches equals (d + 2) ÷ 4 — works over part of the range and fails over the rest. It is exact from 2d to 10d, because there the published table really is a quarter-inch ladder. From 12d it is wrong at every single size: a quarter of an inch out at 12d, a full inch at 16d, and at 60d it claims 15½ inches for a six-inch nail. Use the table.
The diameter is not set by the penny size at all — it is set by the type. A 16d common, a 16d box and a 16d sinker are three different nails: 3½ in by 0.162, 3½ in by 0.135, and 3¼ in by 0.148. The Wood Handbook puts it plainly: box nails are “generally of the same length but slightly smaller diameter”, and cement-coated nails — coolers, sinkers, coated box — are “slightly shorter (1/8 in.) and of smaller diameter”. Anyone reading a penny number as a nail specification is reading half of one.
Where that matters, it matters quantitatively. The Wood Handbook’s equation 8-2 gives the lateral load per nail as p = K D^(3/2), with K from 1,440 to 2,720 in pounds and inches depending on the species’ specific gravity. Because capacity goes as diameter to the power one and a half, the 17% thinner shank of a 16d box nail costs about 31% of its capacity, and at 8d the figure is 25%. The independent check is that a trade source that arrived at its numbers from rated lateral capacities rather than from this formula publishes the common nail’s advantage as “22% to 44% higher” — and every value this page derives from the code diameters lands inside that band. Building codes specify nailing schedules by type as well as by penny size for exactly this reason. This page is a dimensional converter and a capacity estimator, not a structural design tool, and a substitution in a shear wall or a joist hanger is a question for whoever specified it.
Two diameter tables are in circulation and they disagree about common nails. The code family — the ASTM F1667 values that appear in the American Wood Council’s NDS fastener tables — gives 6d 0.113, 8d 0.131, 16d 0.162 and 20d 0.192 in. The older steel-wire-gauge tabulation that most consumer nail charts print gives 0.120, 0.134, 0.165 and 0.203. They agree exactly at 10d and 12d. What almost nobody expects is that they agree on box nails everywhere — so the box nail has one table and the common nail has two. This page carries both and defaults to the code family, because that is the one a schedule is written against.
For most of the world the useful direction is out of the penny system entirely. A metric nail is specified as length × diameter in millimetres, so a 16d common is 88.9 × 4.11 mm, and the nearest commonly stocked metric length is 90. The lengths convert cleanly and the diameters do not line up with anything, which means a metric substitution is always a little thicker or thinner than what it replaced. Three types are offered here and no more: finishing, casing and cooler nails are absent because no source fetched for this page published a diameter for them, and the em dashes in the tables are refusals rather than zeroes. For the neighbouring gauge systems see the wood screw size converter, the drill bit size converter and the wire gauge converter; for the threads that hold things together rather than the shanks that do, the thread pitch and TPI converter.
Frequently asked questions
How long is a 16d nail?
3½ inches, or 88.9 mm. Not 4½, which is what the (d + 2) ÷ 4 rule gives — that rule is exact only from 2d to 10d, where the published table happens to be a quarter-inch ladder. From 12d it is wrong at every size and the error grows: at 60d it says 15½ in for a 6-inch nail.
Are a 16d common and a 16d box the same nail?
No, and this is the fact worth knowing. They are the same length, 3½ in, and different diameters: 0.162 in for the common and 0.135 for the box. A 16d sinker is different again, 3¼ by 0.148, because cement-coated nails are made an eighth or a quarter of an inch shorter and thinner than commons. The penny number tells you the length; only the type tells you the nail.
Does it matter if I use box nails instead of commons?
For structural connections, yes, measurably. Lateral capacity goes as the diameter to the power 1.5, so the 17% thinner shank of a 16d box loses about 31% of its capacity, and a trade source that arrived at its figures from rated capacities rather than from the formula publishes the common’s advantage as 22% to 44%. Building codes specify nailing schedules by type as well as penny size for exactly this reason. This page is a dimensional converter: it will tell you the diameters and the ratio, and it is not a substitute for the schedule or for the engineer who wrote it.
Why do two nail charts give different diameters?
Because there are two tabulations in circulation. The code family — the ASTM F1667 values that appear in the American Wood Council’s NDS fastener tables — gives 6d common 0.113, 8d 0.131, 16d 0.162 and 20d 0.192 in. The older steel-wire-gauge chart that most consumer nail pages print gives 0.120, 0.134, 0.165 and 0.203. They agree exactly at 10d and 12d, and they agree on box nails everywhere. Use the code family for anything load-bearing and expect the box of nails in your hand to say something slightly different from either.
What is the metric equivalent of a 16d nail?
There is no penny system outside North America: a metric nail is specified as length × diameter in millimetres. A 16d common is 88.9 × 4.11 mm, and the nearest commonly stocked metric length is 90 mm. The diameters do not line up tidily at all, so a metric substitution is always a little thicker or thinner than what it replaces.
Why is it called a penny size?
It is the price, in pence, of a hundred nails of that size in 15th-century England — hence the ‘d’, for denarius. A hundred of the smallest cost twopence, so those are 2d. The system is a price list that outlived its currency, which is why nothing about it is systematic and why the (d + 2) ÷ 4 rule only ever fitted part of it.
What is the wire gauge on a nail?
The steel wire gauge of the stock it was drawn from, and it runs backwards — a bigger gauge number is a thinner nail. A 16d common is 8 gauge on the wire-gauge chart and a 2d is 15. It is carried here because it is still printed on boxes, but the diameter in inches or millimetres is the number to compare. For the gauge systems themselves see the wire gauge converter.
Does this page cover finishing and casing nails?
No, deliberately. No source checked for this page published a diameter table for finishing, casing or cooler nails, and a fastener diameter is not something to interpolate. The three types here are the three the fetched sources actually give, and the em dashes in the table are refusals rather than zeroes.
Related calculators
References
- USDA Forest Products Laboratory, Wood Handbook — Wood as an Engineering Material, General Technical Report FPL-GTR-190, chapter 8 “Fastenings” (fetched 27 September 2026). Equation 8-2, the lateral load per nail: p = K D3/2, with p in pounds or newtons, D the nail diameter, and K from its table 8-4 — softwoods 1,440 (50.04) at specific gravity 0.29–0.42, 1,800 (62.55) at 0.43–0.47 and 2,200 (76.45) at 0.48–0.52; hardwoods 1,440 (50.04) at 0.33–0.47, 2,000 (69.50) at 0.48–0.56 and 2,720 (94.52) at 0.57–0.74. The same chapter states the dimensional relationship this page is built on: “bright box nails are generally of the same length but slightly smaller diameter”, and cement-coated nails — “coolers, sinkers, and coated box nails” — are “slightly shorter (3.2 mm (1/8 in.)) and of smaller diameter than common nails”.
- ASTM F1667, Standard Specification for Driven Fasteners: Nails, Spikes, and Staples, and the American Wood Council’s National Design Specification for Wood Construction, appendix L. Both copyrighted; neither table is reproduced. The AWC’s own Design Aid No. 2: Toenailed Connections (fetched 27 September 2026) states the ranges rather than the rows: box nails 0.099 to 0.162 in, common 0.113 to 0.162 in, sinkers 0.099 to 0.148 in.
- turn2engineering, Nail Size Chart: Penny Sizes, Gauge & Dimensions (read 27 September 2026), which names its source as “the AWC NDS Appendix L4 reference” with ASTM F1667/F1667M-21a for tolerances, and prints 8d common 2½ in 0.131, 8d box 2½ in 0.113, 16d common 3½ in 0.162 and 16d sinker 3¼ in 0.148. The MiTek USP fastener catalogue (fetched 27 September 2026) agrees and extends it: 6d 0.113, 10d 0.148, 12d 0.148, 20d 0.192 in.
- All Points Fasteners, Nail Size Chart (read 27 September 2026), the SECOND diameter family and the one most consumer charts print: common nails 2d to 60d, 0.072 to 0.238 in, wire gauges 4 to 15; box nails 3d to 20d, 0.076 to 0.148 in, gauges 10 to 14½. It agrees with the code family on box nails and disagrees with it on commons — 0.120 against 0.113 at 6d, 0.134 against 0.131 at 8d, 0.165 against 0.162 at 16d.
- Journal of Light Construction, Practical Engineering: Box vs. Common Nails (read 27 September 2026), the independent check on the consequence. It gives the diameters — “a common 16-penny nail has a diameter of 0.162 inch, while the 16-penny box nail is 0.135 inch”, and 8d common 0.131 against 8d box 0.113 — and the penalty: “the common nail typically has a rated lateral capacity 22% to 44% higher than that of a box nail of the same penny weight.” The Wood Handbook’s own D3/2 law reproduces that range from the diameters alone, which is how this page proves it rather than quoting it.
- BS 1202-1, Nails. Specification for steel nails. Copyrighted and not fetched; cited by number for the metric designation convention only. Nothing is reproduced from it, and the metric lengths this page snaps to are whole millimetres from the common stocked range.