After the winter I wasted chasing a fake spin number, I made myself actually learn what a launch monitor does and doesn’t measure. It turned out to be the single most useful afternoon I’ve spent on this hobby. So here’s the plain-English version I wish someone had handed me at the start.
Every launch monitor shows you a wall of numbers. The mistake almost everyone makes — me included — is treating them all as equally real. They’re not. Some are measured: the machine directly observed them. Some are calculated: the machine worked them out from the measured ones using math and assumptions. Both can be on the same screen, in the same font, and nothing tells you which is which.
The two camps
Measured numbers are things the unit physically captures. Depending on the technology, that’s usually ball speed, launch angle, launch direction, and — on the better units — spin rate and spin axis, plus club data like club speed and path if it can see the club.
Calculated numbers are derived. Carry distance, total distance, apex height, descent angle, smash factor — these are all computed from the measured inputs plus a physics model and assumptions about the ball, the conditions, and the rollout. Carry distance, for instance, isn’t watched all the way to the ground; it’s projected from the launch conditions the unit measured in the first few feet.
That’s fine — as long as the inputs were measured well. Garbage in, garbage out. If spin was calculated rather than measured, then your carry, your apex and your descent angle are calculations built on top of a guess. The error doesn’t stay put; it compounds down the chain.
Why spin is the one to watch
I keep coming back to spin because it’s the parameter most likely to be calculated on affordable units, and it’s the one that does the most damage when it’s wrong.
To genuinely measure spin, a unit has to either see the ball rotate — a high-speed camera reading the dimples or a marking — or read the subtle radar signature of the spinning ball over distance. That’s hard and it’s expensive. So a lot of budget units measure ball speed (which is comparatively easy) and infer spin from it with a model tuned to an average swing.
For a swing near that average, on a clean strike, the inferred spin can be close enough. But the further you are from average — faster, slower, a steep mishit, a different ball — the more the guess drifts, and you get no warning. That’s exactly how I ended up rebuilding a swing around a spin “problem” that didn’t exist.
The numbers most people get wrong
When I explain this in the forums, a few specific numbers come up over and over as ones people assume are measured when they often aren’t.
Carry distance is the big one. Everyone treats carry like the unit watched the ball land. It didn’t. Carry is projected from the launch conditions measured in the first few feet of flight, then run forward through a physics model. If the spin feeding that model is calculated, your carry is a calculation built on a calculation. That’s why two units can show the same player wildly different carries — they’re not measuring different flights, they’re modelling them differently.
Smash factor is just ball speed divided by club speed. If your unit doesn’t actually measure club speed — and many entry units don’t see the club at all — then either smash factor isn’t shown, or it’s derived from an assumed club speed. A “1.50 smash” on a unit that never saw your club is a tidy-looking number with not much behind it.
Apex and descent angle are downstream of launch and spin, so they inherit whatever error those carry. They’re great for understanding shot shape when the inputs are measured, and misleading when they’re not.
None of this makes those numbers worthless. It makes them numbers you read in context — knowing which are observations and which are projections.
How I read a spec sheet now
When I look at any unit, I ask three things in order:
- What does it measure directly? This is the real list of trustworthy numbers.
- By what method? Doppler radar, photometric camera, or a combination. The method tells you what it’s good and bad at indoors versus outdoors.
- What is it calculating, and from what? Anything not in the measured list is derived — useful, but only as good as its inputs.
A unit that honestly measures ball speed, launch and spin gives you a foundation you can coach off. A unit that measures ball speed and calculates the rest gives you great speed-training data and a story for everything else. Neither is bad. They’re just different tools, and knowing which you’ve got changes what you should and shouldn’t trust them for.
What this means for practice
Here’s the practical rule I live by in the garage now: only let a measured number drive a swing change. Use calculated numbers for trends and feel, not for surgery. If my carry seems short, I look at the measured inputs — ball speed, launch, spin — before I conclude anything, because a “short carry” can just be a calculated number reacting to a soft spin estimate.
It also reframed how I think about price. The big jumps up the ladder aren’t really about features — they’re about what moves from the calculated column into the measured column, and how tight the published accuracy bands are. That’s the thing actually worth paying for.
If you want to see which units keep spin in the measured column instead of guessing it, that’s exactly what I sorted in the best launch monitors for spin guide. Once you can read your screen in two colours — measured and calculated — you stop being fooled by it. That’s the whole game.