📏 Spindle Blank Size Calculator
Enter the finished length and diameter you want, plus a waste margin percentage for tenons, tool marks, and truing up, to get the rough blank size to cut.
One margin, applied to both dimensions
The waste percentage becomes a single multiplier, and the script scales the finished length and the finished diameter by it:
factor = 1 + (margin % / 100)
blank length = finished length × factor
blank diameter = finished diameter × factorThe defaults - a 20 in by 2 in spindle with a 10 per cent margin - give a factor of 1.1, so cut the blank 22.00 in long and 2.20 in across. Push the margin to 25 per cent and it becomes 25.00 by 2.50 in.
Why the length margin needs watching
Diameter waste really is proportional - a fatter blank needs more skim to come round. Length waste is not. What you lose at the ends is a drive spur, a tailstock cup and a parting cut, much the same whatever the spindle. At 10 per cent:
| Finished length | Blank length | Spare per end |
|---|---|---|
| 6 in | 6.60 in | 0.30 in |
| 12 in | 13.20 in | 0.60 in |
| 20 in | 22.00 in | 1.00 in |
| 36 in | 39.60 in | 1.80 in |
Three tenths of an inch is not enough to hold and part off. Decide the spare you want in inches, then raise the percentage until it matches - a 6 in spindle wanting an inch a side needs 33 per cent.
Frequently asked questions
How much waste should I allow on a spindle blank?
Work back from the inches you need rather than picking a percentage. Allow for the drive and tailstock ends plus a parting cut, then set the margin so the spare per end covers it. Short spindles need a far higher percentage.
Does the calculator add the margin to each end?
No. It multiplies the whole length once, so the extra is split across the two ends. A 10 per cent margin on 20 in adds 2 in in total, an inch at each end.
What size square stock do I need for a 2 inch spindle?
At a 10 per cent margin the blank diameter is 2.20 in, so 2¼ in square is the smallest that cleans up. Narrower stock leaves flats.