The field at a glance
| Fact | NTRVL: 4×4 Trainer | Peak Intervals | Norwegian 4x4 Protocol | PEAKVO2 | Zones for Training |
|---|---|---|---|---|---|
| Price | Free · Pro $1.99/mo or $9.99/yr, 7 days free | Free · subscription, 2-week trial | $5.99 once | Free · in-app purchases from $0.99, $17.99 lifetime | Free · PRO subscription |
| On the store since | 16 May 2026 | 5 February 2026 | 1 May 2024 | 18 December 2024 | 13 September 2016 |
| Ratings | No ratings yet | No ratings yet | 4.3 ★ from 50 ratings | 5 ★ from 2 ratings | 4.8 ★ from 19,144 ratings |
| Minimum iOS | iOS 17.0 | iOS 26.0 | iOS 17.6 | iOS 18.0 | iOS 17.6 |
Read from Apple's App Store listing data on 3 September 2026, except PEAKVO2's subscription prices, which are published on the developer's own site. Prices change — check the store before you decide.
The short version
Want a timer you buy once?
Buy Norwegian 4x4 Protocol for $5.99 and stop reading. It is well reviewed and it does the job.
Want many protocols, not one?
PEAKVO2 has twenty-four of them and a $17.99 lifetime unlock.
Want a heart-rate dashboard over everything you do?
Zones for Training has ten years and nineteen thousand ratings behind it. It is not a 4×4 app and does not claim to be.
Want the 4×4 run for you, on whatever you are wearing?
That is what we built. Heart rate from AirPods Pro 3 or your Apple Watch — and the full protocol even with neither. An eight-week programme that shifts when you miss a day, and a recovery trend that moves weeks before VO₂ Max does.
One by one
The app closest to ours: same protocol, same interest in measuring whether the work actually landed.
Read the comparison →A clean, offline 4×4 timer you buy once. The best-reviewed app in this niche, and the cheapest.
Read the comparison →Twenty-four interval protocols in one app, with a lifetime unlock and the cheapest subscription in this group.
Read the comparison →The established heart-rate-zone app: ten years on the store, nearly twenty thousand ratings, and more than seventy workout types.
Read the comparison →The arithmetic behind the zones
Every app on this page draws heart rate zones. Almost none of them say which method they use, and the methods disagree by up to twenty beats. Ours is written out in full:
Where we are weakest
We are the most expensive app in this group, we have no user reviews yet, and we run exactly one protocol. If any of those three is your deciding factor, one of the apps above is a better purchase than ours, and we would rather say so here than have you find out after paying.
Try NTRVL freeOne interval, about twelve minutes, free forever. No heart rate hardware required.