TRIMP Calculator: Banister Training Impulse

TRIMP — training impulse — turns one session into one number by multiplying how long you trained by how hard your heart was working, with the hard part weighted disproportionately. Enter the session length, your average heart rate during it, your resting heart rate and your maximum.

The formula, step by step

1. Heart rate reserve fraction. ΔHR = (exercise heart rate − resting heart rate) ÷ (maximum heart rate − resting heart rate). This puts the session on a 0 to 1 scale between your own floor and your own ceiling, which is why two people with the same average bpm can get very different loads.
2. Intensity weighting. For men, y = 0.64 × e^(1.92 × ΔHR). For women, y = 0.86 × e^(1.67 × ΔHR).
3. TRIMP = duration in minutes × ΔHR × y.

Worked out: someone with a maximum of 200 bpm and a resting rate of 40, training 30 minutes at an average of 130 bpm, has ΔHR = (130 − 40) ÷ (200 − 40) = 0.5625. On the male coefficients the weighting is 0.64 × e^1.08 = 1.885, and the load is 30 × 0.5625 × 1.885 = 32 points. The same session on the female coefficients gives 37, which is a good illustration of why the sex setting is not decorative.

Published papers usually report the result in arbitrary units (AU) rather than points. It is the same number; there is no unit of training in the physical sense, only a scale that is comparable to itself.

Why the weighting is exponential rather than linear

The exponential term is doing the real work. Two sessions of equal duration at different intensities do not cost the same, and the gap widens faster than intensity rises — the weighting was built to follow the blood lactate response to increasing exercise intensity, which curves upward rather than climbing in a straight line.

The practical effect: a 60-minute easy ride does not automatically outrank a 30-minute hard one, which a simple minutes-times-intensity score would have gotten wrong. Run both through the calculator and watch the ordering change.

What the number is for

A single TRIMP score is nearly meaningless on its own. It becomes useful in three ways:

• Summed over a week, it gives a total load you can compare with last week’s, so a jump in volume is visible before it is felt.
• Spread across the days, it feeds training monotony and strain, which asks whether the week had hard days and easy days or seven identical ones.
• Compared against a no-hardware alternative, it keeps you honest — see session RPE, which multiplies a 0 to 10 rating by the minutes and needs no monitor at all.

The blind spots of a heart rate model

Everything that is not cardiovascular. A heavy strength session, a set of short sprints, a climbing session — anything where the muscular cost leads and heart rate lags — scores low on TRIMP relative to how hard it actually was. The inputs also inherit two estimates: your maximum heart rate is probably a formula rather than a measurement (the maximum heart rate calculator shows how wide that error is), and a wrist sensor’s average over an interval session can be well off.

For the wider picture — what internal load means, how the two main methods compare head to head, and what to do with the number once you have it — see what is training load.

This content is informational and does not replace guidance from a coach or clinician.