How to Calculate Your 1RM From a Working Set (kg)
By Twaka team ·
The short answer: use the Epley formula: 1RM = weight × (1 + reps ÷ 30). Take a hard set you actually did, plug in the weight and reps, and you get an estimated one-rep max. It's most useful between 1 and 10 reps. You don't need to test a true single to get a number good enough for planning.
Worked example in kg
Say you squatted 100 kg for 5 reps.
1RM = 100 × (1 + 5 ÷ 30) 1RM = 100 × 1.1667 1RM ≈ 116.7 kg
That's the same example our free one-rep max calculator loads by default. Put in your own set to check your own numbers.
Turning your 1RM into working weights
Once you have an estimate, percentages tell you what to load. From that 116.7 kg estimate:
| % of 1RM | Weight |
|---|---|
| 95% | 110.8 kg |
| 90% | 105 kg |
| 85% | 99.2 kg |
| 80% | 93.3 kg |
| 75% | 87.5 kg |
| 70% | 81.7 kg |
A rough guide to what each band is for:
- 85 to 95%: heavy strength work
- 70 to 85%: most working sets
- 60 to 70%: technique or volume days
Round to the plates you actually have.
How to get a good estimate
Use a real working set, not a warm-up. The formula assumes the set was hard. An easy set gives you a low estimate.
Stay between about 1 and 10 reps. The estimate gets less reliable as reps climb. Above about 10 reps, fatigue and technique drift matter more than the maths.
Use a recent set. Recalculate every few weeks from a recent hard set rather than holding onto an old number.
Same lift, same technique. A paused squat and a touch-and-go squat will give you different numbers. Compare like with like.
Why estimate instead of testing a single?
Testing a true one-rep max is hard on your body and needs a proper warm-up and a spotter. For most people, an estimate from a set of 3 to 5 is far less risky and usually close enough for planning. Only test a true single if your sport needs it.
Where your log comes in
The formula is only as good as the set you feed it. That's where a training log earns its keep. If you write down every hard set, you always have a fresh input.
In Twaka, you write the set the way you'd say it, like Squat 1x5 @ 100kg, and Twaka keeps the sets and weights. Your end-of-workout recap shows how each lift moved since last time, so you can see when it's worth recalculating.
Frequently asked questions
How do I calculate my one-rep max?
Take a hard set you did, then use the Epley formula: weight × (1 + reps ÷ 30). 100 kg for 5 reps gives about 116.7 kg. Our 1RM calculator does it for you.
What is the Epley formula in kg?
1RM = weight × (1 + reps ÷ 30). The formula works in any unit, so kg in gives kg out.
Is it safe to attempt my estimated 1RM?
An estimate is a planning number, not a target to max out on. If you do want to test a single, be well warmed up and have a spotter.
Should I use warm-up sets to estimate 1RM?
No. Use a hard working set. Warm-ups are too easy and will underestimate your max.
Is a 1RM estimate from more than 10 reps accurate?
It gets less reliable as reps climb. Above about 10 reps, treat the number as a rough guide at best. A set of 3 to 5 gives a better estimate.
Should I track effort alongside my sets?
It helps. If you note how hard a set felt, like @8, you'll know which sets are good 1RM inputs. See how to log RPE.
Stop re-typing sets into a calculator
Check your numbers on the free 1RM calculator, then keep the sets that matter in your log. Twaka is a plain-English training tracker for iPhone, now on the App Store. Download Twaka on the App Store.
General training information, not medical advice.
