Three speeds, and only one of them matters
The engine on a walk-behind holds somewhere around 3,000 rpm with the blade engaged. That is the first speed. On a deck driven by a belt, the crank pulley and the spindle pulley are rarely the same size, so the spindle turns at a different rate — with the defaults in the form, a 5.5 in driver against a 6 in driven gives a ratio of 0.917 and a spindle speed of 3,025 rpm. That is the second.
The third is the only one that describes what happens to the grass. A 21 in blade sweeps a circle 65.97 in around, and at 3,025 rpm the tip covers 16,626 feet every minute — 189 mph, or 84 metres a second. Direct-drive walk-behinds skip the middle step entirely: the blade bolts to the crankshaft and the spindle speed is the engine speed.
The pulley measurement everyone gets wrong
Pitch diameter is where the belt actually sits in the groove, and it is smaller than the outside of the pulley. Put a tape across the rims of both pulleys and the ratio comes out close, because both readings are wrong in the same direction, but it is not exact and it gets worse as the pulleys wear — a worn groove lets the belt sit deeper, which changes the effective diameter of that pulley and nothing else.
Belt slip is the other gap. The arithmetic assumes the belt does not move relative to either pulley. A fresh belt on clean pulleys is close to that; a glazed belt on a deck that has been mowing wet grass is not. Both errors push the same way, so a calculated spindle speed is an upper bound rather than a reading. If the answer matters, put a tachometer on the spindle nut and use the measured option in the form.
Blade length is the lever
Tip speed goes up in a straight line with blade length at a fixed rpm, and at 3,025 rpm every extra inch of blade is worth 792 feet per minute at the tip. That is why the length matters so much more than it looks: going from a 16 in blade to a 26 in blade at the same spindle speed takes the tip from 12,668 to 20,586 feet per minute, a difference of 7,918, with nothing about the engine changed at all.
It also runs the other way, which is the part nobody notices. Grinding the tips back over a few seasons shortens the swing. Take half an inch off a 21 in blade and the tip loses 396 feet per minute and the cut loses half an inch of width — small on its own, and the reason a blade that has been sharpened fifteen times behaves differently from the one that came in the box.
What the page will not do
It will not tell you what tip speed your deck should run at. That figure belongs to whoever designed the deck and whoever made the blade, it is bound up with blade thickness and lift geometry and the deck shell, and it is not something a calculator can supply. The target block exists so you can compare against a number from your own machine documentation, and it deliberately stops at the comparison.
The same goes for the pulley the page prints when you enter a target. It is arithmetic — the driven diameter that would land on the number, at the engine speed you gave — and it is not a recommendation to fit one. Changing deck speed changes blade loading, belt life and everything the deck was designed around.
Questions people ask
How do I work out mower blade tip speed?
Multiply pi by the blade length to get the distance the tip travels in one revolution, then multiply by the spindle speed and divide by 12 to get feet per minute. A 21 in blade sweeps 65.97 in per turn, so at 3,025 rpm the tip covers 16,626 feet a minute. The 12 is the step people drop, and dropping it makes the answer twelve times too big.
Is spindle speed the same as engine speed?
On a direct-drive walk-behind, yes, because the blade bolts to the crankshaft. On anything belt driven, no. The ratio is the crank pulley pitch diameter divided by the spindle pulley pitch diameter, so a 5.5 in driver against a 6 in driven turns 3,300 engine rpm into 3,025 at the spindle, and belt slip takes a little more off on top of that.
Why does my new blade cut differently from the old one?
If the old one had been sharpened a dozen times it was shorter, and a shorter blade at the same rpm has a slower tip and a narrower cut. Half an inch off a 21 in blade costs 396 feet per minute at 3,025 rpm and half an inch of width on every pass. Blade thickness, lift shape and how sharp the edge is all change the cut too, and none of those are in this arithmetic.
What tip speed should a mower deck run at?
Whatever the people who built the deck specified, which this page does not know and does not guess. Deck speed is designed alongside the blade, the spindle, the belt and the deck shell, and the figure lives in the machine documentation or with the machine maker. The target block on the page compares against a number you supply and stops there.
Can I measure spindle speed instead of calculating it?
That is the better answer when you can get it. A tachometer at the spindle has the belt slip, the worn grooves and the real governed engine speed already baked in, which is exactly what the calculation has to assume away. Use the measured option in the form and the page skips the pulley arithmetic entirely.