Tractor Horsepower Calculator for Implements

Tractors are sold by horsepower, but implements are rated by what they need: pulling force for a disk or plow, and shaft power for a mower or baler. Matching the two takes a few conversions. This calculator uses the method and data published by University of Georgia Extension engineers, which are based on the ASABE agricultural machinery management standard.

How Implement Horsepower Is Calculated

For pulled implements the calculation has three steps. First find the draft, the force needed to pull the implement through the soil. Then convert draft and speed into drawbar horsepower. Finally scale up to PTO horsepower, because a tractor loses power to wheel slip, rolling resistance and its drivetrain before any of it reaches the hitch.

The draft values come from a table that lists pounds of draft per foot of width, per shank or per row, for each inch of working depth, in clay, loam and sandy soil. Clay is hardest to pull through and sand the easiest, so the same disk needs about a quarter more draft in clay than in sand. Seeding implements and light tillage tools such as harrows and packers do not depend on depth, so the calculator ignores depth for them.

The 375 comes from the definition of horsepower. One horsepower is 33,000 ft·lb per minute, and 1 mph is 88 ft per minute, so 33,000 ÷ 88 = 375 lb·mph.

PTO-driven implements are simpler. Mowers, rakes and tedders are rated in PTO horsepower per foot of width, and balers, forage harvesters and grinders in PTO horsepower per ton per hour of material. Multiply by your width or throughput.

Worked Examples

The first two rows reproduce the University of Georgia's own example. Its text describes the disk as working 8 inches deep, but its arithmetic uses 4 inches, and 4 inches is what gives the published 2,208 lb and 61.8 HP.

Speed matters as much as size. Slowing that disk from 5 to 3.5 mph cuts the requirement by 18.5 PTO horsepower, so a smaller tractor can pull the same implement if you accept covering fewer acres per hour. Draft itself also falls a little at lower speed for most tillage tools, which makes the slow-speed figure slightly conservative.

Brush Hogs, Mowers and Rakes

Forums and dealers usually quote a much lower rule for rotary cutters: about 5 PTO horsepower per foot of cut, so 25 HP for a 5 ft cutter. The standard-based figure is about twice that. A likely explanation is that the 5 HP rule describes the minimum needed to keep a light cutter turning in grass and light brush, while the standard's value is a planning figure for full-width mowing at working speed in heavier growth. For light pasture the lower figure is often enough. For thick brush or stalks, plan nearer the higher figure or expect to slow down.

Always check the cutter maker's minimum and maximum tractor horsepower. The maximum protects the gearbox and driveline from a tractor that can overload them.

Balers, Forage Harvesters and Grinders

Machines that process material are rated per ton per hour of wet material. A small rectangular baler needs about 3.9 PTO HP per ton/hr, so baling 10 tons an hour takes about 39 PTO horsepower. A variable-chamber round baler needs about 6.7 per ton/hr, a corn-silage forage harvester about 12, and a tub grinder 11.3 for alfalfa hay or 16.9 for straw. Drawn machines also need drawbar power to be towed, which these figures do not include.

PTO Horsepower vs Engine and Drawbar Horsepower

Tractor brochures can quote three different numbers, and only one of them suits this calculation:

• Engine horsepower is measured at the crankshaft. The UGA publication notes that a diesel tractor typically delivers 75 to 85% of its rated engine power to the rear PTO.
• PTO horsepower is measured at the PTO shaft. It is the figure most commonly used to size tractors, and the one this calculator returns.
• Drawbar horsepower is what actually pulls the implement. According to the UGA publication, Nebraska Tractor Test results show maximum drawbar power averaging about 85% of maximum PTO power for most tractors.

The soil factors of 1.5, 1.8 and 2.1 bridge that last gap. They allow for the power lost to wheel slip and rolling resistance, which grows from firm to tilled to soft ground.

Lift Capacity, PTO Speed and Ballast

Horsepower is only one of the checks. 1st Products, which publishes torque-meter field measurements for its implements, advises checking the tractor's three-point lift capacity before horsepower. Lift capacity is quoted at about 24 inches behind the lift arms, and the tractor must lift and carry the implement safely over rough ground, not just pull it. The same guide notes that horsepower requirements can vary by as much as 50% between soil conditions, and that heavy, damp soil takes more power than dry, sandy soil.

• PTO speed. PTO shafts run at one of two standard speeds, 540 or 1,000 RPM. The implement's driveline must match the tractor's PTO speed.
• Hitch category. Three-point hitches come in categories sized to tractor power. The implement's pins must match the tractor's category.
• Ballast and tires. Drawbar power only reaches the ground if the tires grip. Front weights or wheel ballast keep a mounted implement from lifting the front axle, and they reduce slip under heavy draft.
• Hydraulics. Hydraulically driven implements need enough oil flow (GPM) from the tractor, which is listed separately from horsepower.

What These Numbers Do Not Include

The results are estimates of the minimum PTO horsepower for the implement alone. Hills, wet patches, heavy residue, hydraulics running a planter's fans or a loader, and air conditioning all add load. Two-wheel-drive tractors also need enough weight on the rear axle to put drawbar power down without excessive slip. Use the result as the floor, then add a margin for your toughest field.

For the difference between engine and wheel power in general, see how horsepower is measured. To compare a tractor's torque rise with its horsepower, use the torque and RPM horsepower calculator.