Estimate steaming time from the thickness of the bend. The common shop rule is about an hour per inch for air-dried stock at full steam; green wood often needs less. Adjust the rate to your own test pieces.
Use this when you are steaming parts for chair backs, boat frames, or bent handles and want a starting time for the steam box.
Enter the least thickness of the section that bends — that is the dimension the heat has to soak through. Start timing when the box is at full steam, not when you light the burner; the warm-up field adds that to the plan.
Make a test bend from the same board before committing good stock. Species, grain straightness, and moisture content move the right time more than any formula.
Steaming works by heating the wood through to its core so the lignin softens. Heat soaks in from both faces, so time is driven by the thinnest dimension of the bending section. The traditional shop rule — about one hour per inch of thickness at full steam for air-dried stock — is a linear rule of thumb, and this page applies it directly.
Green wood already carries a lot of water and usually needs less time; about half is a common starting point. Kiln-dried wood can be steamed for the same time as air-dried, but it often bends poorly regardless.
steam minutes = thickness (in) × minutes per inch
plan minutes = steam minutes + warm-up minutes
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| Condition | Thickness | Min/in | Warm-up | Steam | Plan |
|---|---|---|---|---|---|
| Air-dried | 0.75 | 60 | 20 | 45 min | 1 h 05 min |
| Air-dried | 0.25 | 60 | 20 | 15 min | 35 min |
| Air-dried | 0.5 | 60 | 20 | 30 min | 50 min |
| Air-dried | 1 | 60 | 20 | 1 h 00 min | 1 h 20 min |
| Air-dried | 1.5 | 60 | 0 | 1 h 30 min | 1 h 30 min |
| Green | 1 | 30 | 20 | 30 min | 50 min |
With the calculator defaults (Stock condition = Air-dried; Thickness of the bend (in) = 0.75; Minutes per inch = 60; Steam-box warm-up (min) = 20), the arithmetic runs like this:
steam time = thickness × minutes per inch = 0.75 × 60 = 45 min
plan = steam + warm-up = 45 + 20 = 65 min
Result: 45 min at full steam. 0.75 in (19 mm) stock. With 20 min warm-up, plan about 1 h 05 min from lighting the steamer.
Now change Thickness of the bend (in) to 1.25 and leave everything else alone:
steam time = thickness × minutes per inch = 1.25 × 60 = 75 min
plan = steam + warm-up = 75 + 20 = 95 min
Result: 1 h 15 min at full steam. 1.25 in (31.8 mm) stock. With 20 min warm-up, plan about 1 h 35 min from lighting the steamer.
Going from 3/4 in to 1-1/4 in stock adds 30 minutes at the same rate. Thickness, not length, sets the time — a 6 ft rail and a 1 ft handle of the same section steam for the same period.
Have the form, clamps, and a bending strap laid out before the first piece comes out. Steamed wood starts cooling the moment the box opens, and the working window is short — bend immediately and clamp to the form in one motion.
Straight, clear grain with no runout bends best. A compression strap (a steel band with end blocks) on the outside of the bend keeps the outer fibers from tearing; most breaks start on the convex face.
Over-steaming is not a cure for a stiff piece. If a correctly timed test piece still cracks, look at grain direction, moisture content, and radius before adding more time.
Units: Thickness in inches; time in minutes. The metric thickness is shown for reference.
Where the model stops: It is a linear rule of thumb at full steam (near boiling at the wood surface). It does not model species, box leaks, altitude, or very thick stock where heat soak is slower than linear.
Not a substitute: Steam boxes and boilers are hot. Vent the box, keep the boiler from running dry, and follow the steamer maker’s instructions.
As a starting point for air-dried hardwood, yes — that is the long-standing shop rule. Test bends tell you whether your species and box need more or less.
When steam is flowing steadily out of the box vent, not when you light the burner. The warm-up field covers the time before that.
Thickness of the part that actually bends, in the direction of the bend. For a 3/4 × 2 in slat bent flatwise, enter 0.75.
Soaking can raise surface moisture, but it does not replace steaming for a real bend. Air-dried stock simply bends better than kiln-dried.