You Toolmongers have been holding out on me. How come no one told me — or, perhaps worse, why didn’t I discover before — that there are simple formulas for determining the nominal diameters and clearance holes of (Unified Thread Standard) machine screw sizes 0–14*? All those years I spent looking up that little table, or trying to find that plastic gauge thingy…

Specifically, the nominal diameter in inches for a size N machine screw is: .060″ + (N × .013″), and the clearance hole is: (N +4)/64. For example, a #6 machine screw has a nominal diameter of .060″ + 6 × .013″ = .138″, and its clearance hole is 10/64 = .156″

If you work with really small machine screws like #00 and #000, the formulas also work if you set N to -1 and -2 for screw sizes 00 and 000, respectively.

What other clever, or quick, or back-of-the-envelope calculations should I know about?

*and, yes, I know machine screw sizes 12 & 14 are rare; the next “standard” size above #10 is usually ¼-20.

### 14 Responses to Machine Screw Diameters

1. measure once, cut twice says:

Harbor Freight has a plastic gauge for screws, bolts and nuts.

\$1.99

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=39240

2. rg says:

Nice going, Gordon. You’ve blown our cover.

“You have not much future there. It will happen this way. You may be walking. Maybe the first sunny day of the spring. And a car will slow beside you, and a door will open, and someone you know, maybe even trust, will get out of the car. And he will smile, a becoming smile. But he will leave open the door of the car and offer to give you a lift.”
(– 3 Days of The Condor)

3. MattW says:

Here is one that made me feel pretty dumb when I learned it-

Tap drill size (for approx 75% thread) = nominal size – thread pitch

so tap drill size for M6x1 = 5mm (.197)
for 1/4-20 = .250 – (1/20) = .200

This works for cutting taps, of course, not form taps, so you no longer need a tap drill chart.

4. Chris W says:

Don’t most drill indeces have this info stamped on them? I have never had to look it up. The bit and the information are always together.

5. David Bryan says:

Screw number times .013 plus .060 equals diameter in thousandths.

6. David Bryan says:

Oh, sorry, I didn’t read it and see that you’d already posted that, Gordon.

7. Alan says:

Why make life complicated? The nominal diameter of a size N screw in mm is N.

8. This calculation part is very tough actually.

9. To save on the maths, I find this chart pretty helpful…

http://www.marfas.com/machinescrewtapping.shtml

10. Jay says:

on a metric screw your drill size is the screw size minus the pitch, so a m10x1 would use a 9mm drill to tap. Works every time. If anyone knows any possible way I can find a slotted pan head 4-43 machine screw it will make my week! That’s how long I’ve been searching. Thanks! Great forum!

• Steve says:

Jay, are you sure you don’t need a 4-48? Or is it for something from the 60’s? Send me an email to dellwoodtiresatyahoodotcom and I’ll try to help you.

11. George Dennison says:

The second formula is not to calculate the clearance drill in decimal inches, but to give the fractional drill size to use for the clearance hole.

I worked a table out, and the calcs based on the second formula hit and miss the Close & Free Fit sizes from the first source cited as a reference.

Here’s the data from the cited chart, and the fractional computation from the formula.

Size Close Fit Free Fit Calc Frac
0 0.06350 0.07000 4/64 1/16
1 0.07600 0.08100 5/64 5/64
2 0.08900 0.09600 6/64 3/32
3 0.01040 0.01100 7/64 7/64
4 0.01160 0.01285 8/64 1/8
5 0.01285 0.01360 9/64 9/64
6 0.01440 0.01495 10/64 5/32
8 0.01695 0.01770 12/64 3/16
10 0.01960 0.02010 14/64 7/32
12 0.02210 0.02280 16/64 1/4
14 0.02460 0.02570 18/64 9/32

To me, this actually makes your find even cooler since most of the time we’re going for the fractional drills for clearance holes. It’s also way easy to remember. Clearance is the size plus 4, in 64ths.

I’ve seen a lot of formulas, but never these.

I always have wondered how & why the old timers came up with the goofy sizing of machine screws, and this shows why.

12. George Dennison says:

damn proportional fonts!! messed up my table!
let’s see if this is any better:

Size Close Fit Free Fit Calc Frac
0 0.06350 0.07000 4/64 1/16
1 0.07600 0.08100 5/64 5/64
2 0.08900 0.09600 6/64 3/32
3 0.01040 0.01100 7/64 7/64
4 0.01160 0.01285 8/64 1/8
5 0.01285 0.01360 9/64 9/64
6 0.01440 0.01495 10/64 5/32
8 0.01695 0.01770 12/64 3/16
10 0.01960 0.02010 14/64 7/32
12 0.02210 0.02280 16/64 1/4
14 0.02460 0.02570 18/64 9/32

13. George Dennison says:

I surrender…