# Heart Rate Zones, Explained @icon(book-open) @category(health) > Your watch says "Zone 3" — this is where that number comes from, one step at a time. Four short steps, each with the arithmetic and a picture, ending in your own five zones. Change your age or resting rate at any point and every step re-derives. #training #running #health #explainer # Start here @icon(play) @skin(spec-sheet) @density(compact) age = 38 // years 15..90 @label(Age) @icon(calendar) resting heart rate = 58 // bpm 35..110 @label(Resting HR) @icon(moon) @note(Taken lying down, before getting out of bed) /// Every zone number on every fitness watch comes from **two** numbers and **one** line of arithmetic. Here they are. # Engine @hidden _max heart rate = round(208 - 0.7 * age) _heart rate reserve = max(1, max_heart_rate - resting_heart_rate) _zb(p) = round(resting_heart_rate + heart_rate_reserve * p) _lo = [_zb(0.5), _zb(0.6), _zb(0.7), _zb(0.8), _zb(0.9)] _hi = [_zb(0.6), _zb(0.7), _zb(0.8), _zb(0.9), _zb(1)] _names = ["Recovery", "Aerobic", "Tempo", "Threshold", "VO₂ max"] _fills = ["fill-seq-1", "fill-seq-2", "fill-seq-3", "fill-seq-4", "fill-seq-5"] _ops = [1, 1, 1, 1, 1] _k70 = round(resting_heart_rate + heart_rate_reserve * 0.7) _m70 = round(max_heart_rate * 0.7) # 1 · The ceiling @icon(arrow-up-to-line) /// Everything is measured against your **maximum heart rate** — the fastest your heart can beat. Almost nobody has measured theirs, so it gets estimated from age. The best-fitting estimate is Tanaka's: `208 − 0.7 × age`. max hr = max_heart_rate // bpm @hero @label(Your estimated max) @icon(arrow-up-to-line) @note(Tanaka: 208 − 0.7 × age, the best-fitting age formula) the ceiling = svg(``) // @label(Estimated, not measured) @wide /// That amber band is the honest part: any age formula carries roughly **±10 bpm** of person-to-person spread. Two 38-year-olds can both be normal and 20 bpm apart. It is a starting point, not a fact about you. # 2 · The floor, and what is left @icon(arrow-down-to-line) /// Your heart never slows below its resting rate, so the bottom of the scale is unreachable. Subtract it and what remains is the range you can actually train in — the **heart rate reserve**. reserve = heart_rate_reserve // bpm @hero @label(Heart rate reserve) @icon(arrow-left-right) @note(max − resting: the only stretch of the scale you can train in) the floor = svg(``) // @label(What is actually trainable) @wide # 3 · One line of arithmetic @icon(calculator) /// A zone is just a **slice of the reserve**, added back on top of your resting rate. That is the whole Karvonen method: `zone edge = resting + reserve × percent`. Run it six times and you have five zones. zone 3 floor = _zb(0.7) // bpm @hero @label(Zone 3 starts at) @icon(calculator) @note(resting + reserve × 0.70 — the diagram below shows the same sum) slice = svg(``) // @label(Zone 3 is one tenth of the reserve) @wide # 4 · Why two watches disagree @icon(git-compare) /// Here is the part that trips everybody up. The **other** common method skips your resting rate entirely: `max × percent`. Both call it "70%", and they give different heart rates. karvonen 70 = _k70 // bpm @label(Karvonen at 70%) @note(resting + reserve × 0.70) percent of max 70 = _m70 // bpm @label(Plain % of max at 70%) @note(max × 0.70 — resting rate never enters the sum) disagreement = _k70 - _m70 // bpm @hero @label(Same 70%, this far apart) @icon(triangle-alert) @note(Exactly resting HR × 0.30 — the gap shrinks to zero at 100%) two models = svg(``) // @label(The same five labels, two different sets of numbers) @wide /// Notice the two bars end together and separate as you go left. The gap is exactly `resting HR × (1 − intensity)` — so it barely matters for a sprint and matters a lot for an easy day. If an easy run feels harder than it should, this is often why. # Your five zones @icon(flag) @skin(spec-sheet) @density(compact) /// Everything above, applied to your numbers. Karvonen, off a Tanaka max estimate. zone 1 = "{_lo[0]:plain:0}–{_hi[0]:plain:0} bpm" // @label(1 · Recovery) @note(You could sing. Warm-ups and easy days — hold it for hours.) zone 2 = "{_lo[1]:plain:0}–{_hi[1]:plain:0} bpm" // @label(2 · Aerobic) @note(Full sentences. Where endurance is actually built — most of your week.) zone 3 = "{_lo[2]:plain:0}–{_hi[2]:plain:0} bpm" // @label(3 · Tempo) @note(Short sentences. Comfortably hard, and easy to drift into by accident.) zone 4 = "{_lo[3]:plain:0}–{_hi[3]:plain:0} bpm" // @label(4 · Threshold) @note(A few words at a time. Hard but holdable for 30–60 minutes.) zone 5 = "{_lo[4]:plain:0}–{_hi[4]:plain:0} bpm" // @label(5 · VO₂ max) @note(No talking. Intervals of 3–8 minutes, nothing longer.) ladder = svg(``) // @label(Your ladder) @wide /// One habit beats every refinement here: keep easy days genuinely easy — under {_hi[1]:plain:0} bpm — so the hard days can actually be hard.