Drill a reamed hole about 0.1–0.2 mm under size for holes up to 5 mm and 0.2–0.3 mm under for 5 to 20 mm (roughly .004–.012 in). That stock, measured on the diameter, is what the reamer removes: enough to cut cleanly, not so much that it chatters or goes oversize.
This chart gives the stock allowance by hole size and material, the pre-drill size for common inch reamers from 1/8 to 1 in and metric reamers from 3 to 25 mm, and the H7 limits for the metric sizes worked out from the ISO 286 IT7 values. Each table shows the stock the drill actually leaves, so you can see how far from ideal a stock drill size is. Switch inch/mm to read the inch table in millimetres, and use the search boxes to find a size. Values are for machine reaming; hand reamers want much less stock.
Reaming stock allowance by hole size
| Reamer diameter | Stock on diameter (mm) | Stock on diameter (in) |
|---|---|---|
| Up to 3 mm (≈1/8 in) | 0.05–0.15 | .002–.006 |
| Over 3 to 5 mm (≈1/8–3/16 in) | 0.10–0.20 | .004–.008 |
| Over 5 to 20 mm (≈3/16–3/4 in) | 0.20–0.30 | .008–.012 |
| Over 20 to 32 mm (≈3/4–1-1/4 in) | 0.30–0.40 | .012–.016 |
| Over 32 to 50 mm (≈1-1/4–2 in) | 0.40–0.50 | .016–.020 |
| Hand reaming, any size | 0.05–0.10 | .002–.004 |
Typical figures from reamer makers' catalogues and common shop practice for HSS and carbide machine reamers. The stock is on the diameter; half of it comes off each side. The reamer calculator uses the traditional 0.010 / 0.015 / 0.020 in allowances, which sit at the top of this range and also work well in steel and aluminium.
Adjusting the stock for the material
| Material | Where to work in the band | Why |
|---|---|---|
| Aluminium, brass, free-cutting steel | Middle to upper end | Too little stock lets soft metal smear; sharp, polished flutes help. |
| Mild and alloy steel (annealed) | Middle | The default case in the tables. |
| Stainless steel, titanium, nickel alloys | Middle; never below the lower end | The reamer must cut under the layer the drill work-hardened, or it rubs and glazes. |
| Cast iron | Upper end | Cuts freely; a light cut can leave the drill marks. |
| Hardened steel (over about 45 HRC), carbide reamer | Lower end | Cutting pressure is high; follow the carbide reamer maker. |
| Plastics | Upper end | Holes close up after reaming; check after the part cools and relaxes. |
Inch reamers: pre-drill sizes
| Reamer | Decimal | Stock band | Ideal drill | Inch drill | Stock left | Metric drill | Stock left |
|---|---|---|---|---|---|---|---|
| 1/8" | .12503.17 | .004–.0080.10–0.20 | .11913.02 | #31 .12003.05 | .00500.13 | 3.0 mm | .00690.17 |
| 3/16" | .18754.76 | .004–.0080.10–0.20 | .18164.61 | #14 .18204.62 | .00550.14 | 4.6 mm | .00640.16 |
| 1/4" | .25006.35 | .008–.0120.20–0.30 | .24216.15 | C .24206.15 | .00800.20 | 6.1 mm | .00980.25 |
| 5/16" | .31257.94 | .008–.0120.20–0.30 | .30467.74 | N .30207.67 | .01050.27 | 7.7 mm | .00940.24 |
| 3/8" | .37509.52 | .008–.0120.20–0.30 | .36719.32 | U .36809.35 * | .00700.18 | 9.3 mm | .00890.22 |
| 7/16" | .437511.11 | .008–.0120.20–0.30 | .429610.91 | 27/64" .421910.72 * | .01560.40 | 10.9 mm | .00840.21 |
| 1/2" | .500012.70 | .008–.0120.20–0.30 | .488212.40 | 31/64" .484412.30 * | .01560.40 | 12.4 mm | .01180.30 |
| 9/16" | .562514.29 | .008–.0120.20–0.30 | .550713.99 | 35/64" .546913.89 * | .01560.40 | 14.0 mm | .01130.29 |
| 5/8" | .625015.88 | .008–.0120.20–0.30 | .613215.57 | 39/64" .609415.48 * | .01560.40 | 15.6 mm | .01080.28 |
| 11/16" | .687517.46 | .008–.0120.20–0.30 | .675717.16 | 43/64" .671917.07 * | .01560.40 | 17.2 mm | .01030.26 |
| 3/4" | .750019.05 | .008–.0120.20–0.30 | .738218.75 | 47/64" .734418.65 * | .01560.40 | 18.8 mm | .00980.25 |
| 13/16" | .812520.64 | .012–.0160.30–0.40 | .800720.34 | 51/64" .796920.24 | .01560.40 | 20.3 mm | .01330.34 |
| 7/8" | .875022.22 | .012–.0160.30–0.40 | .863221.93 | 55/64" .859421.83 | .01560.40 | 21.9 mm | .01280.33 |
| 15/16" | .937523.81 | .012–.0160.30–0.40 | .925723.51 | 59/64" .921923.42 | .01560.40 | 23.5 mm | .01230.31 |
| 1" | 1.000025.40 | .012–.0160.30–0.40 | .988225.10 | 63/64" .984425.00 | .01560.40 | 25.1 mm | .01180.30 |
No rows match that search.
Inch drills include fractional, number and letter sizes (ASME B94.11M). "Ideal drill" is the reamer size minus the target stock. * No inch drill lands inside the band: from 3/8 to 3/4 in the choice is a letter drill that leaves a little less (U for 3/8) or a drill 1/64 under that leaves .0156 in, which is fine in steel and aluminium but on the heavy side for stainless; the metric drill is closer.
Metric reamers: H7 limits and pre-drill sizes
| Reamer | H7 hole limits (mm) | H7 deviation (µm) | Stock band (mm) | Pre-drill | Stock left (mm) |
|---|---|---|---|---|---|
| Ø3 H7 | 3.000 – 3.010 | 0 / +10 | 0.05–0.15 | 2.9 mm | 0.10 |
| Ø4 H7 | 4.000 – 4.012 | 0 / +12 | 0.10–0.20 | 3.8 mm | 0.20 |
| Ø5 H7 | 5.000 – 5.012 | 0 / +12 | 0.10–0.20 | 4.8 mm | 0.20 |
| Ø6 H7 | 6.000 – 6.012 | 0 / +12 | 0.20–0.30 | 5.8 mm | 0.20 |
| Ø7 H7 | 7.000 – 7.015 | 0 / +15 | 0.20–0.30 | 6.8 mm | 0.20 |
| Ø8 H7 | 8.000 – 8.015 | 0 / +15 | 0.20–0.30 | 7.8 mm | 0.20 |
| Ø9 H7 | 9.000 – 9.015 | 0 / +15 | 0.20–0.30 | 8.8 mm | 0.20 |
| Ø10 H7 | 10.000 – 10.015 | 0 / +15 | 0.20–0.30 | 9.8 mm | 0.20 |
| Ø11 H7 | 11.000 – 11.018 | 0 / +18 | 0.20–0.30 | 10.8 mm | 0.20 |
| Ø12 H7 | 12.000 – 12.018 | 0 / +18 | 0.20–0.30 | 11.8 mm | 0.20 |
| Ø13 H7 | 13.000 – 13.018 | 0 / +18 | 0.20–0.30 | 12.7 mm | 0.30 |
| Ø14 H7 | 14.000 – 14.018 | 0 / +18 | 0.20–0.30 | 13.7 mm | 0.30 |
| Ø15 H7 | 15.000 – 15.018 | 0 / +18 | 0.20–0.30 | 14.7 mm | 0.30 |
| Ø16 H7 | 16.000 – 16.018 | 0 / +18 | 0.20–0.30 | 15.7 mm | 0.30 |
| Ø17 H7 | 17.000 – 17.018 | 0 / +18 | 0.20–0.30 | 16.7 mm | 0.30 |
| Ø18 H7 | 18.000 – 18.018 | 0 / +18 | 0.20–0.30 | 17.7 mm | 0.30 |
| Ø19 H7 | 19.000 – 19.021 | 0 / +21 | 0.20–0.30 | 18.7 mm | 0.30 |
| Ø20 H7 | 20.000 – 20.021 | 0 / +21 | 0.20–0.30 | 19.7 mm | 0.30 |
| Ø22 H7 | 22.000 – 22.021 | 0 / +21 | 0.30–0.40 | 21.7 mm | 0.30 |
| Ø24 H7 | 24.000 – 24.021 | 0 / +21 | 0.30–0.40 | 23.7 mm | 0.30 |
| Ø25 H7 | 25.000 – 25.021 | 0 / +21 | 0.30–0.40 | 24.7 mm | 0.30 |
No rows match that search.
H7 = lower deviation 0, upper deviation IT7 from ISO 286-1: 10 µm up to 3 mm, 12 µm over 3 to 6, 15 µm over 6 to 10, 18 µm over 10 to 18 and 21 µm over 18 to 30 mm (the same values as the ISO limits and fits calculator). Drills in 0.1 mm steps; above 20 mm some suppliers only stock 0.5 mm steps, so check the stock it leaves.
Reaming speed and feed compared with drilling
| Drilling | Reaming (rule of thumb) | |
|---|---|---|
| Cutting speed | 100% (your drilling speed for the material) | About 1/2 to 2/3 of the drilling speed (HSS); carbide reamers run faster |
| Feed per revolution | 100% | About 2 to 3 times the drilling feed |
| Coolant | Normal flood or through-tool | Plenty of coolant; cutting oil helps finish in steel and stainless |
| Retract | Rapid is fine | Feed out, or stop the spindle first; never reverse the spindle in the hole |
Slow the speed and raise the feed: a reamer that rubs at high speed and light feed chatters, glazes the bore and wears quickly. If holes come out oversize, reduce the speed or check runout; if they come out undersize or tapered, the reamer may be worn, or the stock may be too small. The reamer calculator has a Size Correction tab for this.
How to use this chart
- Find the reamer size. For a metric H7 hole, note the limits you have to hit.
- Check the stock band and move up or down within it for the material (table above).
- Pick the pre-drill size from the table and look at the stock it really leaves.
- Spot drill, then drill. For holes that must be accurately positioned, drill undersize, bore or interpolate to the pre-ream size, then ream: the reamer follows the existing hole.
- Ream at about 2/3 of the drilling speed and 2–3 times the drilling feed, then gauge the first hole.
Worked example: Ø10 H7 in mild steel
Limits 10.000–10.015 mm. Band for 5–20 mm: 0.2–0.3 mm. Drill 9.8 mm (0.20 mm stock), ream 10 H7. If you drilled at 1,000 rpm and 0.15 mm/rev, start the reamer near 650 rpm and 0.3–0.45 mm/rev.
Worked example: 1/2 in reamer in 6061 aluminium
Band .008–.012 in. A 31/64 drill (.4844 in) leaves .0156 in, which is fine in aluminium. A 12.4 mm drill (.4882 in) leaves .0118 in if you prefer to stay in the band.
Shop tips
- Runout kills reamed hole size. Check the reamer in the holder; a floating holder helps on a manual machine or lathe tailstock.
- Do not stop the feed with the reamer cutting in the hole; dwell marks the bore.
- Blind holes: drill deeper than the reamed depth so the chamfer and chips have somewhere to go, and use a spiral-flute reamer that pulls chips up.
- Reamers cut slightly oversize in most setups. For a tight hole, a reamer made a few microns under the nominal size may be needed; check the first part.
Related calculators and charts
Sources: ISO 286-1 (IT7 values and H7 limits), ASME B94.11M (drill sizes). Stock allowances and speed and feed ratios are typical values from reamer makers' catalogues and common shop practice, not from a standard. Check critical holes against the reamer maker's data.