TOOL 02 — DOWEL & PEG

Dowel & Peg Fit Planner

Match a drill to a measured dowel, peg or axle.

What kind of fit?

Glued joints want the hole snug; a removable peg needs finger clearance; an axle must spin freely without slop.

Glued dowel joint

Removable peg

Rotating axle or wheel

Measured diameter

Measure the actual part with callipers — nominal dowels commonly run over or under size. 1.00 to 25.00 mm, up to two decimals.

Dowel depth in this part (optional)

How far the dowel sits into the hole, 5 to 100 mm whole millimetres. Adds a drilling-depth suggestion with 2 mm of glue relief.

Advanced — override the fit allowances

Minimum allowance

Signed, −0.50 mm to +2.00 mm, two decimals. Must be less than the maximum.

Maximum allowance

Leave blank to use the planned fit above: glued dowel joint +0.00 mm to +0.20 mm; removable peg +0.20 mm to +0.50 mm; rotating axle or wheel +0.50 mm to +1.00 mm.

WORKED EXAMPLE

9/32 inclearance +0.79 mm

Fit window
Value	Measurement
Measured diameter	6.35 mm
Window low	6.85 mm
Window high	7.35 mm

Candidate drills

Drill	Clearance	Pick
7.0 mm	+0.65 mm
9/32 in	+0.79 mm	Recommended

Planned as a rotating axle or wheel: the hole is allowed to run 0.50 mm to 1.00 mm over the measured diameter. Those allowances are a workshop rule of thumb and can be overridden above.
Timber moves with humidity and dowels are rarely truly round — test the fit in scrap first.

This is the tool's worked example, not your figures: a 6.35 mm axle measured with callipers, planned as a rotating fit. Load it into the form with the example button, or enter your own and press the calculate button to replace it.

This planner needs JavaScript to calculate in your browser — nothing is ever sent anywhere. Without it, the method works on paper: add your fit's allowance window to the measured diameter, then pick from the drill table in “How this works” the drill inside that window (the smallest for a glued joint, the one nearest the window's middle for a peg or axle). The panel above shows the full worked example: a 6.35 mm measured axle gives a window of 6.85 to 7.35 mm and a 9/32 in drill at +0.79 mm clearance.

How this works

Inputs

The fit (glued dowel, removable peg, or rotating axle or wheel), the measured diameter (1.00–25.00 mm, two decimals) taken with callipers from the actual part, and — for glued joints — an optional dowel depth for a drilling-depth suggestion. The advanced fields override the default allowance window when your work needs a different fit.

Method

Everything is worked in whole micrometres (thousandths of a millimetre), so no float noise can move a recommendation. The measured diameter becomesd = halfUp(diameter × 1000) µm, and the fit window is:

window = [d + min allowance, d + max allowance]workshop rule of thumb — adjustable above

Default allowances by fit
Fit	Minimum	Maximum
Glued dowel joint	+0.00 mm	+0.20 mm
Removable peg	+0.20 mm	+0.50 mm
Rotating axle or wheel	+0.50 mm	+1.00 mm

Every stocked drill inside the window is a candidate. A glued joint takes the smallest candidate (snuggest hole); a peg or axle takes the candidate nearest the middle of the window, ties going to the smaller drill. When no drill lands in the window, the tool reports the nearest drill below it (tighter than planned) and above it (looser than planned) with a warning for each, so you can decide which compromise the job can carry.

The drill table — metric 0.5 mm steps and imperial 32nds
Drill	Diameter
1.0 mm	1.000 mm
1.5 mm	1.500 mm
2.0 mm	2.000 mm
2.5 mm	2.500 mm
3.0 mm	3.000 mm
1/8 in	3.175 mm
3.5 mm	3.500 mm
5/32 in	3.969 mm
4.0 mm	4.000 mm
4.5 mm	4.500 mm
3/16 in	4.763 mm
5.0 mm	5.000 mm
5.5 mm	5.500 mm
7/32 in	5.556 mm
6.0 mm	6.000 mm
1/4 in	6.350 mm
6.5 mm	6.500 mm
7.0 mm	7.000 mm
9/32 in	7.144 mm
7.5 mm	7.500 mm
5/16 in	7.938 mm
8.0 mm	8.000 mm
8.5 mm	8.500 mm
11/32 in	8.731 mm
9.0 mm	9.000 mm
9.5 mm	9.500 mm
3/8 in	9.525 mm
10.0 mm	10.000 mm
13/32 in	10.319 mm
10.5 mm	10.500 mm
11.0 mm	11.000 mm
7/16 in	11.113 mm
11.5 mm	11.500 mm
15/32 in	11.906 mm
12.0 mm	12.000 mm
12.5 mm	12.500 mm
1/2 in	12.700 mm
13.0 mm	13.000 mm

Rounding

The measured diameter rounds to the nearest micrometre on entry; imperial drills are converted at 25.4 mm to the inch exactly and rounded to the nearest micrometre. Clearances display as signed millimetres to two decimals, halves rounding away from zero. The drilling-depth suggestion is the entered depth plus 2 mm of glue relief — a workshop rule of thumb, so the joint closes rather than bottoming out.

Limitations

Timber moves with humidity, and dowels are rarely truly round — a calliper reading is one diameter of several.
Fluted and smooth dowels behave differently in the same hole; flutes carry glue and forgive a snugger fit.
Always test the fit in scrap from the same board before drilling the work.
This is a planning aid, not an engineering or safety certification.

Worked example

DW-2: rotating axle, measured 6.35 mm. d = 6350 µm; the axle window adds +0.50 mm to +1.00 mm, giving 6.85–7.35 mm. Two drills land inside: 7.0 mm (+0.65 mm) and 9/32 in (7.144 mm, +0.79 mm). The window's middle is 7.10 mm; 9/32 in sits 0.044 mm from it against 0.100 mm for the 7.0 mm drill, so 9/32 in is recommended at +0.79 mm clearance. This is the reading shown in the result panel until you calculate your own.

Sources

Nothing on this page cites an external source. The allowance windows are labelled a workshop rule of thumb and can be overridden above; the drill table lists commonly stocked metric and imperial sizes with their exact converted diameters. The site-wide bibliography lives on the sources page.