Reaming Allowance Calculator
Get the recommended reaming allowance and pre‑drill size for a target hole. Choose metric or imperial, select your tool and workpiece material (same list as the speeds & feeds calculator), and optionally choose a fit target to bias the allowance tighter or looser. Results update live.
How it works
Allowance is the stock left in the hole for the reamer to remove. Too little can burnish or chatter; too much can oversize the hole.
The calculator estimates a range using hole size bands plus material & tool factors, then outputs the suggested pre‑drill range:
Pre‑drill = Final Diameter − Allowance
Typical shop guidance (approx.): up to 6 mm → 0.05–0.15 mm; 6–12 mm → 0.10–0.25 mm; 12–20 mm → 0.20–0.35 mm; 20–50 mm → 0.30–0.60 mm. The app adjusts these with material/tool and fit target.