Why implement makers rate in PTO horsepower, not engine HP. Measured across 197 tractors the median loss is 18 percent, and smaller machines lose the most.
Every rotary cutter, tiller and post-hole digger you will ever shop for states a minimum horsepower, and that number always means PTO horsepower. The number on the tractor's hood is engine horsepower, and it is always higher. Buying against the wrong one is the most common way people end up with an implement their tractor cannot turn.
Engine horsepower is measured at the flywheel, before anything is driven. PTO horsepower is measured at the power take-off shaft, after the engine has already turned the hydraulic pump, the charge pump, the cooling fan, the alternator and the transmission's own internal losses. Nothing is wrong with the tractor; the difference is simply everything the engine does before it reaches the shaft.
The implement manufacturer rates in PTO horsepower because that is the power actually available to spin its gearbox. A 25 HP tractor that delivers 19 PTO HP is a 19 HP tractor as far as a rotary cutter is concerned.
The figure repeated across the internet is 20 to 30 percent, and it is too high. All 197 tractors in this database publish both numbers, so the loss can be measured rather than estimated.
The median loss is 18 percent. Roughly two thirds of these machines lose less than 20 percent, and only a third fall inside the 20-to-30 band that gets quoted as typical. This page said 20 to 30 until August 2026; it was wrong against the site's own data and has been corrected.
The loss is not a flat percentage across the class. Grouped by engine size, the median loss falls steadily as machines get bigger: 21.5 percent below 25 engine HP, 18.0 percent from 25 to 45, and 16.9 percent above 45.
The reason is that the fixed overhead does not scale down. A hydraulic pump, a cooling fan and an alternator draw broadly similar power whether they sit on a 20 HP sub-compact or a 60 HP utility tractor, so on the smaller machine that overhead is a larger share of a smaller total. This is why sub-compact owners are so often surprised: the proportional bite is worst exactly where there is least to spare.
If you only know engine horsepower - a used listing, a hood decal, a dealer advert - a reasonable working estimate is engine HP x 0.82, and x 0.78 if the machine is under 25 engine HP. Treat that as a planning figure and nothing more.
Do not buy an implement on it. Every tractor page on this site states the manufacturer's published PTO figure with a link to the document it came from, and the operator's manual on your own machine states it too. An estimate is for narrowing a shortlist; a published figure is for spending money.
An implement's stated minimum is the power needed to turn it under load, not the power needed to do the job well. A 5-foot rotary cutter rated at a 25 HP minimum will turn on exactly 25 PTO HP and will bog in heavy grass, because the rating assumes the blades are moving air, not standing brush.
Where an implement also publishes a maximum, that number matters more than the minimum. Exceeding it can shear the gearbox or twist the driveline, and unlike being underpowered it fails expensively rather than slowly. Gearboxes are rated for a torque range in both directions.
The compatibility checker on this site compares an implement's minimum PTO horsepower against the tractor's published PTO figure, never its engine figure, and states the margin in the result so you can see how much headroom you actually have.
In order of reliability: the operator's manual specification table; the manufacturer's current spec sheet or brochure for that model year; the model page on this site, which links the document each figure came from. Dealer listings and aggregator sites are the least reliable - they routinely quote engine horsepower in a field labelled PTO, and they carry over figures from adjacent models in the same series.
One caution on model years: PTO output can change between generations under the same model name, usually because emissions equipment changed. Match the document to your machine's year rather than assuming the current brochure describes a ten-year-old tractor.
PTO HP is the power available at the rear power-take-off shaft after losses through the transmission, hydraulic pump, alternator, and cooling fan. Typically PTO HP is 80 to 90 percent of gross engine HP on gear-drive tractors, and 75 to 85 percent on hydrostatic (HST) models because the hydrostatic transmission consumes additional power.
Always use PTO HP for PTO-driven implements like rotary cutters, tillers, finish mowers, and post hole diggers. Implement minimum HP requirements published by the manufacturer assume PTO HP, not engine HP. Using engine HP will overstate your tractor's real working capacity and risk under-powering the implement.
A 10 to 20 percent margin above the implement's minimum PTO HP is a good rule for typical conditions. For tall or wet grass, dense brush, or frozen ground, target 25 percent or more. A tractor running near its PTO HP limit will bog down, overheat the hydrostatic system, and accelerate clutch wear.
Hydrostatic transmissions use hydraulic pumps and motors instead of a mechanical clutch, and these consume 5 to 10 percent more power than a gear-drive setup. Manufacturers publish a lower HST PTO HP figure to reflect this. When sizing implements for an HST tractor, always use the HST PTO HP number from the spec sheet.
Published 2026-01-20, last updated 2026-08-24. Every figure quoted in this guide is taken from manufacturer documentation and linked in the text, and the tractors and attachments it references are records in the FitTractor compatibility database.