Rehab Load Calculator (1RM Percentages)

What a percentage chart is for

Strength programs are usually written in percentages rather than absolute weights, because a percentage means the same thing to everyone. "Three sets of eight at 75%" describes the same relative effort whether you lift 40 kg or 140 kg. To turn that into a number you can load onto a bar, you need one reference point: your one-rep max, the heaviest weight you could lift once with good form.

Most people never actually test a true one-rep max, and in a rehab context, deliberately loading up to a genuine maximum is often not something clinicians want you doing at all. That is where estimating comes in.

How the estimate works

This calculator uses the Epley formula, published by Boyd Epley in 1985 and one of the most widely used equations of its kind. It is simple: take the weight you lifted, multiply it by one plus your rep count divided by thirty. Lift 60 kg for 8 reps and the formula estimates a max around 76 kg.

The formula has a well-known limitation. It is most reliable for sets of roughly one to ten reps, and its accuracy drops off noticeably above that. Once you are past ten or twelve reps, other things start dominating: how well-conditioned you are, how your form holds up as you tire, even how hard you are willing to push. Two people with identical maxes can produce wildly different rep counts at 60%. That is why this calculator caps the input at twelve reps and flags anything above ten.

It is also worth knowing that Epley is one of several formulas. Brzycki, Lombardi, Mayhew, O'Conner, Wathen, and Lander all exist, and they give different answers from the same input. For sets in the one-to-ten range they typically land within about five percent of each other, which is close enough to be useful and far enough apart to remind you this is an estimate.

What the three bands mean

Roughly 85% and above, in the 3 to 6 rep range, is generally described as the zone for maximal strength. The adaptation is largely about the nervous system getting better at recruiting muscle fibres and coordinating a heavy effort. Rest periods here are long, often three to five minutes, because the point is quality of effort rather than accumulated fatigue.

Roughly 65 to 85%, in the 6 to 15 rep range, is the range most associated with muscle size. It balances meaningful mechanical load against enough total work to drive growth. Rest is usually shorter, often around a minute to ninety seconds.

Roughly 40 to 60%, above about 15 reps, is generally described as muscular endurance and light reconditioning. In a rehab context this is often where things start: light enough that a joint and its surrounding tissues are not being challenged hard, heavy enough that the muscle is actually working.

One nuance is worth adding, because it has changed how these bands are read. Recent reviews of the research have found that muscle growth can occur across a surprisingly wide span of loads, from quite light to quite heavy, provided the sets are taken close to the point where you could not do another rep. The traditional bands are still a reasonable organising framework. They are just less rigid than they were once presented as being.

Where this fits in rehab

Loading after an injury or an operation is not the same problem as loading a healthy athlete, and this is the part where a chart stops being enough. Tissue that is repairing has its own tolerance. The right load might be far below what your uninjured side could handle, and it might change from week to week. Programs often start in the light, high-rep range and progress from there based on how the joint responds, not on a preset percentage schedule.

Your physical therapist is choosing loads with information a calculator does not have: what was operated on, how it looks and moves, how it responded to last week's session. If their prescription and this chart disagree, follow the prescription.

Used sensibly, though, a percentage chart is genuinely useful. It gives you a way to understand why a program is written the way it is, a sane starting point when you are cleared to add load, and a common language for talking to your physical therapist or coach about intensity.

This tool provides educational information only. It is not medical advice. Always consult your doctor or physical therapist.

Frequently asked questions

How accurate is the Epley estimate?

For sets of roughly one to ten reps, the common estimation formulas generally land within about five percent of each other and of a tested max. Above ten reps, accuracy drops off because endurance and technique fatigue start affecting the rep count more than raw strength does.

Why does the chart only go up to 90%?

Loads above 90% sit in a very narrow band where small errors in an estimated max become large errors in practice, and where the margin for a technique breakdown is smallest. For a general reference tool, and especially for anyone in rehab, stopping at 90% is the more cautious choice.

Why do I get fewer reps at 75% than the chart says?

Because the rep numbers are typical figures, not rules. Actual reps at a given percentage vary widely between individuals and between exercises. Many people manage more reps at a given percentage on leg exercises than on upper-body ones. If your numbers differ consistently, trust your own numbers.

Should I use this to pick my rehab weights?

Not on your own. Loading after an injury or surgery depends on what was injured, how it is healing, and how it responded to recent sessions. Your physical therapist has that information. Use the chart to understand the program you are given, not to write one.

Can I test a real one-rep max instead of estimating?

That is a question for your clinician if you are in rehab, because a genuine maximum attempt is a high-load event. Many programs avoid it entirely and work from estimates or from higher-rep testing instead.

Why does the calculator round down?

So the suggested load never exceeds what the percentage implies. Rounding down by a couple of kilos costs you almost nothing in training effect and keeps the chart on the cautious side.

Sources

• National Strength and Conditioning Association (NSCA) — Training Load Chart: nsca.com
• Topend Sports — 1 Rep Max Calculator and formula reference (Epley, Brzycki, Lombardi, Mayhew, O'Conner, Wathen, Lander): topendsports.com
• American College of Sports Medicine — Resistance Training Guidelines update (2026): acsm.org
• Progression Models in Resistance Training for Healthy Adults, ACSM position stand: researchgate.net
• Schoenfeld BJ, Grgic J et al. — Loading Recommendations for Muscle Strength, Hypertrophy, and Local Endurance: A Re-Examination of the Repetition Continuum: vuir.vu.edu.au
• Robergs RA, Gordon T et al. — RM strength prediction analysis, Journal of Strength and Conditioning Research: unm.edu
• Oxford Reference — strength-endurance continuum: oxfordreference.com