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Nut size lookup
Pick a nut size for its wrench, height and tap drill — or pick a wrench to find the nut it fits.
★ REFERENCE CHART · CONSTRUCTION · FASTENERS
Nut Size Chart: Wrench Size & Tap Drill
A 3/8-16 nut takes a 9/16 in wrench (14.29 mm), stands 21/64 in (8.33 mm) tall, and needs a 5/16 in tap drill (7.94 mm). Every hex nut from 1/4-20 to 1-8 is in the table below, with the wrench size, the height and the tap drill — pick a size in the calculator and the reverse lookup tells you the nut from the wrench in your hand.
The full chart
Standard hex nuts to ASME B18.2.2, coarse (UNC) threads. Wrench size is the distance across the flats — the size stamped on the wrench. Tap drill is the drill size for cutting the matching internal thread at about 75% thread engagement.
| Nut size | Wrench (in) | Wrench (mm) | Height (in) | Height (mm) | Tap drill | Tap (mm) |
|---|---|---|---|---|---|---|
| 1/4-20 | 7/16 | 11.11 | 7/32 | 5.56 | #7 (0.201 in) | 5.11 |
| 5/16-18 | 1/2 | 12.70 | 17/64 | 6.75 | F (0.257 in) | 6.53 |
| 3/8-16 | 9/16 | 14.29 | 21/64 | 8.33 | 5/16 (0.312 in) | 7.94 |
| 7/16-14 | 5/8 | 15.88 | 3/8 | 9.52 | U (0.368 in) | 9.35 |
| 1/2-13 | 3/4 | 19.05 | 7/16 | 11.11 | 27/64 (0.422 in) | 10.72 |
| 5/8-11 | 15/16 | 23.81 | 9/16 | 14.29 | 17/32 (0.531 in) | 13.49 |
| 3/4-10 | 1-1/8 | 28.57 | 21/32 | 16.67 | 21/32 (0.656 in) | 16.67 |
| 1-8 | 1-1/2 | 38.10 | 7/8 | 22.22 | 7/8 (0.875 in) | 22.22 |
The worked example, line by line
Defaults: nut 3/8-16 — 3/8 in diameter, 16 threads per inch.
Line 1 — the wrench. A 3/8-16 hex nut measures 9/16 in (14.29 mm) across the flats, so a 9/16 in wrench or socket fits it. That is 1.5 times the nominal diameter — the usual rule for nuts up to 1/2 in.
Line 2 — the height. The nut is 21/64 in (8.33 mm) tall, a little under its 3/8 in diameter. Height is what sets how much thread the nut grips: about 7/8 of the diameter for a standard hex nut.
Line 3 — the tap drill. To cut a 3/8-16 internal thread, drill 5/16 in (7.94 mm) first. The drill is smaller than the bolt by one thread depth on each side: 3/8 in minus 2 × (1 ÷ 16 in × 0.54) lands on 5/16 in.
Reverse: a 9/16 in wrench in your hand fits a 3/8-16 nut; a 3/4 in wrench fits 1/2-13; a 1-1/8 in wrench fits 3/4-10; a 7/16 in wrench fits 1/4-20.
Every branch, checked
1/4-20: 7/16 in wrench (11.11 mm), 7/32 in tall (5.56 mm), #7 tap drill (0.201 in, 5.11 mm). The smallest nut in the table — and the one whose tap drill is a numbered size, not a fraction.
1/2-13: 3/4 in wrench (19.05 mm), 7/16 in tall (11.11 mm), 27/64 in tap drill (0.422 in, 10.72 mm). The workhorse size: half-inch bolts hold most of what a house and a car need held.
3/4-10: 1-1/8 in wrench (28.57 mm), 21/32 in tall (16.67 mm), 21/32 in tap drill (0.656 in, 16.67 mm). Note the coincidence: the height and the tap drill are both 21/32 in here.
1-8: 1-1/2 in wrench (38.10 mm), 7/8 in tall (22.22 mm), 7/8 in tap drill (0.875 in, 22.22 mm). The biggest nut in the table — the wrench is half as wide again as the bolt.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Wrench size | across-flats distance, ASME B18.2.2 | 9/16 in (14.29 mm) for a 3/8-16 nut. |
| Nut height | standard hex height, ASME B18.2.2 | 21/64 in (8.33 mm) for a 3/8-16 nut. |
| Tap drill | drill for ~75% thread engagement | 5/16 in (7.94 mm) for a 3/8-16 thread. |
| 1/4-20 nut | table row | 7/16 in wrench, 7/32 in tall, #7 tap drill. |
| 1/2-13 nut | table row | 3/4 in wrench, 7/16 in tall, 27/64 in tap drill. |
| 3/4-10 nut | table row | 1-1/8 in wrench, 21/32 in tall, 21/32 in tap drill. |
| 1-8 nut | table row | 1-1/2 in wrench, 7/8 in tall, 7/8 in tap drill. |
| Reverse: 9/16 in wrench | reverse lookup | Fits a 3/8-16 nut. |
| Reverse: 3/4 in wrench | reverse lookup | Fits a 1/2-13 nut. |
| Reverse: 1-1/8 in wrench | reverse lookup | Fits a 3/4-10 nut. |
| Reverse: 7/16 in wrench | reverse lookup | Fits a 1/4-20 nut. |
Who uses this
- The home mechanic: they learn a 1/2-13 nut takes a 3/4 in wrench (19.05 mm) — so they grab the right socket the first time, and they check the 27/64 in tap drill before re-threading a stripped hole.
- The DIYer mid-project: they learn a 3/8-16 nut takes a 9/16 in wrench — so they know which wrench to buy for the deck hardware, and they check the 5/16 in tap drill for the pilot holes.
- The machinist: they learn the 1/4-20 tap drill is a #7 (0.201 in, 5.11 mm), not a fraction — so they know the numbered drill to reach for, and they check the 7/8 in tap drill for the 1-8 thread.
- The wrench-in-hand puzzler: they learn a 1-1/8 in wrench fits a 3/4-10 nut — so they can name the nut from the tool, and they check the 7/16 in wrench against the 1/4-20 nut before guessing.
Accuracy & limitations
Sizes are the ASME B18.2.2 standard for regular hex nuts, coarse (UNC) threads — the sizes stamped on the hardware-store bin. Real nuts vary a little by class of fit and manufacturer: a wrench that is a snug fit on one brand can be loose on another, and worn or rusted nuts measure larger than the table. Tap drill sizes assume about 75% thread engagement, the usual target for a strong thread in steel; softer materials (aluminum, plastic) often use a slightly larger drill for easier tapping. Fine threads (UNF, e.g. 3/8-24) take different tap drills and are not in this table. When the fit matters — a brake line, a load-bearing bolt — measure the actual part, don’t trust the table alone. For bolt sizing, see stud size chart.
Cite this calculatorBright Side Kit. “Nut Size Chart: Wrench Size & Tap Drill.” brightoolbox.com, 2026. Standard: ASME B18.2.2 hex nuts, coarse (UNC) threads; wrench size = across-flats distance; tap drill for ~75% thread engagement. Defaults: 3/8-16 → 9/16 in wrench (14.29 mm), 21/64 in tall (8.33 mm), 5/16 in tap drill (7.94 mm). Rows: 1/4-20 → 7/16 in (11.11 mm), 7/32 in (5.56 mm), #7 (0.201 in, 5.11 mm); 5/16-18 → 1/2 in (12.70 mm), 17/64 in (6.75 mm), F (0.257 in, 6.53 mm); 7/16-14 → 5/8 in (15.88 mm), 3/8 in (9.52 mm), U (0.368 in, 9.35 mm); 1/2-13 → 3/4 in (19.05 mm), 7/16 in (11.11 mm), 27/64 in (10.72 mm); 5/8-11 → 15/16 in (23.81 mm), 9/16 in (14.29 mm), 17/32 in (13.49 mm); 3/4-10 → 1-1/8 in (28.57 mm), 21/32 in (16.67 mm), 21/32 in (16.67 mm); 1-8 → 1-1/2 in (38.10 mm), 7/8 in (22.22 mm), 7/8 in (22.22 mm). Reverses: 9/16 in → 3/8-16; 3/4 in → 1/2-13; 1-1/8 in → 3/4-10; 7/16 in → 1/4-20.
Bright Side Kit. "Nut Size Chart: Wrench Size & Tap Drill." https://brightoolbox.com/nut-size-chart/ Accessed October 25, 2026.Frequently asked questions
What wrench size do I need for a 3/8 nut?
A 9/16 in wrench (14.29 mm). A 3/8-16 hex nut measures 9/16 in across the flats. The nut is also 21/64 in (8.33 mm) tall, and the matching tap drill is 5/16 in (7.94 mm).
What wrench fits a 1/2 inch nut?
A 3/4 in wrench (19.05 mm). A 1/2-13 nut is 3/4 in across the flats, 7/16 in (11.11 mm) tall, and its tap drill is 27/64 in (0.422 in, 10.72 mm).
What tap drill do I use for a 1/4-20 thread?
A #7 drill (0.201 in, 5.11 mm). Numbered drills cover the small sizes: #7 is the standard 75%-thread tap drill for 1/4-20. For 5/16-18 it is an F drill (0.257 in, 6.53 mm); for 7/16-14 it is a U drill (0.368 in, 9.35 mm).
How do I read a nut size like 1/4-20?
The first number is the nominal diameter in inches (1/4 in) and the second is the threads per inch (20). A bigger second number means finer threads: 1/4-20 is coarse, 1/4-28 is fine. The wrench size comes from the first number only.
Why is the wrench bigger than the nut size?
Because the wrench grips the nut across the flats, and the flats span wider than the bolt diameter. A 3/8 in nut is 9/16 in across the flats — 1.5 times its diameter. The ratio holds for most sizes in the table.
What wrench do I need for a 1 inch nut?
A 1-1/2 in wrench (38.10 mm). A 1-8 nut is 1-1/2 in across the flats, 7/8 in (22.22 mm) tall, and its tap drill is 7/8 in (0.875 in, 22.22 mm).
Can I use a metric wrench on an inch nut?
Usually not well. An 11 mm wrench is 0.11 mm smaller than a 7/16 in nut (11.11 mm), so it will not fit at all; a 14 mm wrench is 0.29 mm smaller than a 9/16 in nut (14.29 mm) and may jam on. Close is not close enough with hex fasteners — a loose wrench rounds the corners. Use the inch size from the chart.
What is the difference between coarse and fine threads?
Coarse (UNC) threads have fewer threads per inch and are the default in this chart — 3/8-16, 1/2-13, 1/4-20. Fine (UNF) threads, like 3/8-24, grip better under vibration but strip more easily and take different tap drills. This chart covers coarse threads only.