Training zones calculator

Your FTP gives the power zones. Your threshold heart rate gives the heart rate zones. The two will disagree during a session, and that disagreement is useful.

## What the zones are for

Zones exist so that "ride hard" becomes something you can repeat on purpose. Every one of them stresses a different system, and the reason a training plan moves between them is that doing only one makes you good at only that.

The seven power zones below are the ones in general use, from Andrew Coggan's work, and they are set as percentages of FTP. The heart rate zones are set from threshold heart rate, not from maximum.

## Why not percentages of maximum heart rate

Because almost nobody knows their maximum.

The familiar formula — 220 minus your age — is a population average with a standard deviation somewhere around ten to twelve beats. For a forty-year-old it predicts 180, and the real figure across a room of forty-year-olds will run from roughly 160 to 200. Build your zones on that and you can easily be a whole zone out in either direction, which is worse than having no zones at all.

Threshold heart rate, by contrast, is something you can measure this week: ride the last twenty minutes of a hard hour and take the average. It also moves with your fitness in a way that maximum does not, which makes it the more useful anchor.

## Power and heart rate disagree, and that is information

If you ride a steady tempo effort and watch both numbers, the power will sit still and the heart rate will climb. That drift is normal — it is cardiac drift, driven mostly by heat and fluid loss, and indoors it is pronounced because you are sitting inside your own waste heat.

This is why the two sets of zones are not interchangeable:

• Power is what you are doing. It is instant, it does not drift, and it does not care how you slept.
• Heart rate is what it is costing you. It lags a minute or two behind, it rises through a long effort at constant power, and it will be flatly wrong on very short intervals because your heart simply cannot respond fast enough.

For intervals under about three minutes, ignore heart rate entirely. By the time it reaches the right number the effort is over. Zone 6 and 7 in the table have no heart rate range for exactly that reason.

A useful habit: if your heart rate is ten beats high for the same power, you are probably tired, hot, dehydrated or coming down with something. If it is ten beats low and the power feels fine, you are fresh. Neither means the session is wrong, but both are worth noticing.

## Where the useful work happens

Most riders spend too much time in zone 3. It is the natural pace of an unstructured ride — hard enough to feel productive, not hard enough to drive much adaptation, and tiring enough to compromise the next session. The old joke is that it is too hard to be easy and too easy to be hard.

The two zones that reliably repay effort are at the ends:

Zone 2 builds the aerobic base, and the only way to get enough of it is volume. It should feel almost boringly easy; if you can't hold a conversation, you are in zone 3 again.

Zone 4 and 5 raise the ceiling, and they work because they are hard. Four by eight minutes at threshold, or five by four minutes at VO2 max, with real recovery between. These are the sessions that hurt and move the number.

The sessions in between are not useless, but they should be a deliberate choice rather than where you drift to.

## A note on indoor zones

If you tested indoors, use indoor zones. If you tested outdoors, expect to be unable to hit the top of them on the trainer, and do not treat that as failure. The honest fix is to hold two sets of numbers, or simply to accept that an indoor session at the same perceived effort will be a few per cent lower in watts.