One Rep Max (1RM) Calculator

Testing a true one-rep max can be risky without experience or supervision, but you can get a solid estimate from a lighter set taken close to failure. This calculator estimates your one-rep max from the weight and reps you lifted, and breaks it down into a percentage table for planning your training weights across different phases of a programme.

How to use the one-rep max calculator

Enter the weight you lifted and the number of reps you completed in that set, ideally a set taken close to muscular failure with good form. The calculator estimates your one-rep max and shows a percentage table, so you can quickly see what weight corresponds to 50%, 70%, 90% and other common training percentages of your estimated maximum.

How the calculation works

This calculator uses the Epley formula, one of the most widely used 1RM estimation methods: one-rep max equals the weight lifted multiplied by 1 plus reps divided by 30. This formula reflects the general pattern that lifting a lighter weight for more reps roughly corresponds to a predictable single maximum lift, though the estimate becomes less reliable for very high rep sets, where muscular endurance starts to matter more than pure strength.

Worked example

Lifting 100kg for 5 reps gives an estimated one-rep max of 100 × (1 + 5/30) = 116.7kg. From that figure, common training percentages work out at 58.3kg at 50%, 81.7kg at 70%, 93.3kg at 80%, and 105kg at 90%, giving a ready-made set of target weights for different phases of a training programme.

Other 1RM formulas you might see

The Epley formula used here is not the only method for estimating a one-rep max. The Brzycki formula is another widely used option, calculated as weight lifted divided by 1.0278 minus 0.0278 times reps. For the same 100kg lift for 5 reps used above, the Brzycki formula gives an estimate of around 112.5kg, slightly lower than the Epley estimate of 116.7kg. Neither formula is definitively “correct”, and different formulas tend to agree closely at low rep counts but diverge more as reps increase, which is part of why estimates from very high rep sets should be treated with more caution than those from lower rep sets.

Staying safe when testing your strength

Whether estimating or directly testing strength, a few general practices reduce injury risk. A thorough warm-up, including lighter warm-up sets building up towards your working weight, helps prepare muscles and joints for a heavy effort. Using a spotter or safety bars for exercises like the barbell bench press or squat adds a safety net if a lift does not go to plan. Prioritising good form over chasing a bigger number matters too, since a heavier lift with breakdown in technique is both less accurate as a strength indicator and carries a higher injury risk than a slightly lighter lift performed well.

Using percentages to structure training

Different training goals are generally associated with different percentage ranges of 1RM. Heavier work in the 85% to 100% range, performed for low reps, tends to be associated with building maximal strength. Moderate loads in the 65% to 85% range, for moderate rep counts, are commonly used for building muscle size. Lighter loads below around 65%, for higher reps, are often used for muscular endurance, technique work, or as part of a warm-up building towards a heavier working set. These are general patterns rather than fixed rules, and a full programme will typically blend several rep ranges across a training cycle rather than sticking to just one.

Your estimated 1RM is also a useful way to track progress over time. Retesting or re-estimating every few weeks or months and comparing the figure lets you see whether a training programme is working, independently of the specific weights and reps used in any individual session, since the underlying 1RM estimate accounts for both.

Why your strength varies day to day

Do not be surprised if an estimate from one session looks noticeably different from another, even without a formal training block in between. Sleep quality, stress levels, nutrition, hydration, how recently you last trained that muscle group, and even the time of day can all genuinely affect how much weight you can lift on a given day. A single estimate is a useful snapshot, but tracking your estimated 1RM over several sessions across a few weeks gives a much more reliable picture of your actual underlying strength trend than any one session in isolation.

Common mistakes to avoid

A common mistake is estimating from a set that was not close to failure, which understates your true strength, or from a set with breakdown in technique, which can overstate it since form deteriorating under fatigue does not reflect a genuine strength limit. Try to base your estimate on a recent set of good-quality reps taken close to, but not necessarily all the way to, failure.

Another mistake is treating the estimate as exact rather than a useful approximation, particularly for very high rep sets, where 1RM formulas generally become less reliable. It is also worth retesting periodically as you get stronger, since your estimated 1RM from months ago will not reflect your current strength.

Related calculators

If running is also part of your training, our running pace calculator works out your pace and predicts race finish times. To plan nutrition around your training, our TDEE calculator estimates your daily calorie needs, and our BMR calculator works out your resting metabolic rate as a starting point for that calculation. Strength and cardiovascular training work well together as part of a balanced overall training routine.