People arrive at this question after leaving WHOOP, or while considering it, and want to know which Apple Watch number to look at instead. The honest answer is that Apple never built one, but it does record everything needed to build one.
What strain is actually trying to measure
Cardiovascular load: how much stress your heart and circulatory system absorbed today. WHOOP computes it largely from time spent in heart rate zones across the entire day, not just during logged workouts, and expresses it on a 0 to 21 logarithmic scale.
The logarithmic part matters and is widely misunderstood. Moving from 17 to 18 represents far more additional work than moving from 7 to 8. A day at 14 is not twice as hard as a day at 7.
The genuinely good idea underneath it is counting the whole day. A stressful commute, a flight of stairs taken fast, an afternoon on your feet, none of these get logged as workouts and all of them cost something.
What the Apple Watch gives you instead
Training Load. Effort ratings from 1 to 10 per workout, with the last 7 days compared against the last 28. Workouts only, so the whole-day component of strain is missing entirely. Detail in Training Load explained.
The Move ring. Active energy in kilocalories. Closer to a whole-day measure, but energy is not stress: body mass drives it heavily, and a long easy walk can out-earn a brutal interval session.
Continuous heart rate. Sampled all day and written to Apple Health. This is the actual raw material for a strain score, and Apple simply does not aggregate it into one.
The problem strain does not solve
A single whole-body cardiovascular number cannot tell a leg day from a back day. Heavy lifting is the clearest case: a demanding set of squats barely moves your heart rate and taxes your legs enormously, so it scores low on every strain model in existence while being one of the most costly things you did that week.
That is not a flaw in Apple’s implementation or WHOOP’s. It is a limit of measuring load through a wrist sensor at all. The argument in full is in why your wearable has no idea which muscles are tired.
What to actually watch instead
If you have left WHOOP and miss the number, the useful substitutes are less elegant and more informative:
Time in zones across the week, rather than one aggregate. It tells you the shape of your training, not just the volume. Getting the zones right first matters, which is zone 2 heart rate, and why yours is probably wrong.
The ratio of this week to your last four. The idea underneath Training Load, and the one with the best evidence behind it for managing risk.
Load against recovery. Strain in isolation answers half a question. The same hard week means different things depending on what your overnight numbers did afterwards.
Weighing the strap against the watch you own? That comparison is WHOOP vs Apple Watch.