Gearbox reduction ratio: converting motor input RPM to actual output RPM
A maintenance tech assumed a '20:1 gearbox' meant the output shaft spun 20 times faster than the input, and was confused why the conveyor was running so slowly — the reduction ratio works in the opposite direction, slowi
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
A maintenance tech assumed a '20:1 gearbox' meant the output shaft spun 20 times faster than the input, and was confused why the conveyor was running so slowly — the reduction ratio works in the opposite direction, slowing things down by that factor.
Here is the conversion I wish I had taped to the fridge before that went sideways.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
Output RPM = Input RPM ÷ gear ratio — for a 20:1 gearbox, 1 input RPM = 0.05 output RPM
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Multiplying input RPM by the gear ratio number instead of dividing — a 'reduction' gearbox by definition slows the output down, so the ratio needs to divide the input speed, not multiply it.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in input RPM (20:1 gearbox).
- Confirm the target unit is really output RPM, not a similar-looking one.
- Apply the factor: Output RPM = Input RPM ÷ gear ratio — for a 20:1 gearbox, 1 input RPM = 0.05 output RPM.
- Round only at the end, to the precision your tool or spec supports.
Worked example
A motor spins at 1,750 RPM and drives a 20:1 reduction gearbox.
1750 input RPM (20:1 gearbox) × 0.05 = 87.5 output RPM
That's 87.5 output RPM — round only to the precision your tool or spec actually needs.
Flip the factor once as a sanity check. If you do not land near the starting number, you mixed definitions or inverted the direction.
Quick reference table
| input RPM (20:1 gearbox) | Calculation | output RPM |
|---|---|---|
| 1 | 1 × 0.05 | 0.05 |
| 2 | 2 × 0.05 | 0.1 |
| 5 | 5 × 0.05 | 0.25 |
| 10 | 10 × 0.05 | 0.5 |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
Correctly predicting output RPM is essential for matching a gearbox to the actual speed a downstream conveyor, mixer, or pump needs to run at — getting the direction of the ratio backwards produces a wildly wrong estimate.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Divide input RPM by the gear ratio to get output RPM — a 1,750 RPM motor through a 20:1 reduction gearbox outputs 87.5 RPM, not 35,000.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
Try the converters mentioned in this article
FAQ
What is the exact input RPM (20:1 gearbox) to output RPM factor here?+
Output RPM = Input RPM ÷ gear ratio — for a 20:1 gearbox, 1 input RPM = 0.05 output RPM
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Multiplying input RPM by the gear ratio number instead of dividing — a 'reduction' gearbox by definition slows the output down, so the ratio needs to divide the input speed, not multiply it.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-16 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.