Inch Pounds to Foot Pounds for Motor Torque Ratings

When the Motor Is Rated in Inch-Pounds and the Load Is Not

Motor and gearbox catalogues sprawl across three imperial scales. Fractional-horsepower units and steppers are listed in ounce-inches, geared units in inch-pounds, and the machine designer's load calculation almost always comes out in pound-feet. Before you can say whether a drive is big enough, the whole datasheet has to be pulled onto one scale.

1 in·lbf = 0.08333333 ft·lbf, because twelve inch-pounds make a pound-foot exactly. A gearmotor rated 1,000 in·lbf delivers 1,000 ÷ 12 = 83.33 ft·lbf at its output shaft.

Three Scales, One Family

Sixteen ounce-inches make an inch-pound and 192 make a pound-foot, so a stepper quoted at 288 oz-in is 18 in·lbf or 1.5 ft·lbf. All three integers are exact, which makes the ladder easy to sanity-check by hand.

Ratio Multiplies, Losses Subtract

A reduction raises torque roughly in proportion to its ratio and drops speed by the same factor, but never at full value. A worm set may hand back only 60–80 % of the ideal figure, and that efficiency belongs in the calculation before any unit conversion.

Which Rating Are You Reading?

Continuous, intermittent, peak and stall torque can differ by a factor of three on the same nameplate. Converting the wrong line yields a perfectly correct pound-foot figure for a duty the motor cannot actually sustain.

Normalising a Datasheet Column

Enter the catalogue figure

Type the in·lbf rating and read the pound-foot equivalent straight away. Values here are often large round numbers, and the exact twelfth keeps them clean: 600 in·lbf is precisely 50 ft·lbf.

Pull the oz-in models onto the same scale

Fractional units listed in ounce-inches are handled from the same page: open the searchable dropdown and choose in·ozf on the input side, and the pound-foot column keeps its meaning across the whole product range.

Build the comparison table

Copy each converted value as a bare number and paste it beside the model code in your selection sheet, so competing gearmotors are judged on a single column rather than three.

Turn the load figure back the other way

Your own load calculation probably lands in pound-feet. Type it into the ft·lbf side, or press the swap arrows, to see what to look for in a catalogue that indexes everything by inch-pounds.

Gearmotor Output Ratings on the Larger Scale

Output torque bands you meet across a geared-product catalogue, with the sort of reduction that produces them. Real ratings depend on frame size, duty cycle and gearbox type, so treat these as the shape of the range rather than a selection guide.

Drive type	Rated output (in·lbf)	Same rating (ft·lbf)	Typical reduction
Compact planetary gearmotor	100	8.33	10:1
Actuator or positioner drive	250	20.83	25:1
Light industrial gearmotor	500	41.67	30:1
Right-angle worm gearmotor	1,000	83.33	60:1
Winch or hoist drive	1,800	150.00	100:1
Conveyor head drive	3,000	250.00	150:1

A worked case: a 1/2 hp motor at 1,750 rpm produces 5252 × 0.5 ÷ 1750 = 1.50 ft·lbf, or 18 in·lbf. Put it through a 30:1 reduction at 90 % efficiency and the output shaft turns at about 58 rpm carrying roughly 40.5 ft·lbf — 486 in·lbf, which is the number the catalogue would print.

Exact Twelfths, No Drift

Because the ratio is an integer, a long selection sheet converts without accumulating the rounding error a decimal factor would leave behind.

Ounce-Inches in the Same Menu

Stepper and small servo ratings can be entered directly by switching the input dropdown, so the tiny end of the catalogue needs no separate tool.

Large Values Stay Readable

Thousands are separated by a space in the output, so a five-digit conveyor rating is legible at a glance instead of being a wall of digits.

Drive Sizing Questions

Which torque number on a motor nameplate should I be converting?

The one that matches your duty. Continuous rating is what the motor can hold all day without overheating and is the figure to size a steady load against. Peak or intermittent ratings cover acceleration and short bursts. Stall or locked-rotor torque is a limit condition rather than an operating point, and sizing a conveyor against it is how drives end up cooking.

Why are small motors rated in ounce-inches at all?

Resolution. A NEMA 17 stepper holding roughly 0.39 N·m is about 3.45 in·lbf or 0.29 ft·lbf — awkward decimals either way, but a comfortable 55 oz-in. The ladder is exact at both steps, sixteen ounce-inches to the inch-pound and 192 to the pound-foot, so nothing is lost climbing it.

Can I just multiply motor torque by the gear ratio?

Only as a first estimate. Multiply by the ratio, then multiply again by the gearbox efficiency — often above 90 % for a single planetary stage, considerably less for a worm set, and lower again at high ratios or on a cold start. Speed divides by the ratio cleanly; torque does not multiply cleanly, and the gap between the two is exactly the losses.

The datasheet lists horsepower but no torque. How do I get one?

Use torque in pound-feet = 5252 × hp ÷ rpm, taking rpm at the shaft you care about. The answer arrives in pound-feet, so a fractional-horsepower result is often more useful once turned into inch-pounds, where it lines up with how the catalogue indexes those models.

What is the service factor doing to the rating I compare against?

It is the headroom the application demands. A smooth, steady load may need a factor near 1.0, while a shock-loaded crusher or a frequently reversing drive can call for 1.75 or more. Multiply your calculated load torque by that factor first, and only then look for a gearmotor whose converted rating clears the result.