1RM calculator — does it work?

There are a lot of variables that go into what may be your true 1 rep max. Beyond how much strength you have, psychological factors can go into determining your success at it (more on this later). In this article, we explore Strength Level’s 1RM calculator and how you can use it for your own strength training.

What is 1 rep max?

In the simplest terms, 1RM refers to the maximum about of weight you can lift for one repetition. Simple as that. This is most used to determine how strong a person is and it’s very important to lifting-based sports such as powerlifting.

Beyond 1RM, you can also have 2RM, 3RM, etc. This is refers to the heaviest weight where you can perform xx reps continuously. Because 1 rep max is technically challenging, it may not be suitable for beginners. You should also not attempt to do 1RM without a spotter or safety arms, etc in case of failure.

Understanding RPE aka rate of perceived exertion

RPE, rate of perceived exertion
This table demonstrates how you to interpret a RPE score.

This is probably also a good time to introduce another concept: RPE aka rate of perceived exertion. In essence, it’s a scale of 0 to 10 to determine how much effort it took to perform particular set of exercise is. RPE 10 refers to 100% effort. In other words, you cannot perform another repetition. Conversely, RPE 0 = no kick.

In my opinion, RPE works better when it’s closer to 10. As per, it’s easier to gauge when you are at your limit versus trying to figure out how much effort is an exercise at RPE of 0 to 5-6. I find that when effort is fairly low, the boundaries between each rating is fairly blur.

Effectively, RPE is a good way of looking at how close you are to max effort aka RPE 10.

To frame this within the context of strength testing, 1RM equals to RPE 10 when performing a single rep. 6RM equals to RPE 10 to perform six repetitions.

How does the 1RM calculator work?

1rm calculator, strength training
It doesn’t get simpler than that! Put in the weight you have been lifting as well as the number of repetitions you can perform. The website will generate your max repetitions for you.

Case study: Bench press

5RM is 65kg. That works out to 1RM of 73.1kg. I didn’t try because it’s not important to me. During the same session after ample rest, my 8 rep max was 60kg. Quite close to the estimated 58.5kg by the calculator.

For 40kg, I was able to do 15 reps. According to the calculator, it should be closer to 24 reps!!

So obviously the next thing to do was to give it a shot and see if I could actually get to 24 reps.

I couldn’t.

I got to 18 reps and burst out laughing cos of how ridiculous the whole situation was. I kept going, without resting, to push for 24. I only made it till 20. Do note that I tried this at the end of the workout so it may not be totally accurate.

Re-testing 24-rep max the next day

Granted I am not feeling the freshest the next day, I still wanted to see how close I could get to 24-rep max with 40kg. I expected myself to fail for the first round, and to treat that as a warm up. Surprisingly, I did get to rep 24 though the last few reps were a fair grind.

To be fair, the capacity of performing 24 reps seem to be there but the sng-ness (aka soreness) is a strong 10/10. It’s been 10 minutes and I can still feel my soreness in my right deltoid. Definitely not the best experience!

On that note also, I could probably squeeze a couple more reps in if I could overwrite the sng sensation, which I didn’t. It’s really more of a function of do I really want to go through this suffering than I genuinely couldn’t push another 1-2 reps out. So with that in mind, it’s a 10/10 effort from an overall effort (mental or psychological counts too) and probably a RPE 9 from a biological, mechanical perspective.

Set 2: 50kg for 14 reps

There was no way I was going to put myself through another set of 24 but I did want to see how far I can push this. Based on a weight of 50kg, I should be aiming for 14-15 reps.

I ended up at 13 and that’s probably also a good RPE 9. I could potentially push out rep 14 but I wasn’t 100% confident. Since I was alone, and not particularly fresh since I already did chest yesterday, I decided to leave it at rep 13.

How to use 1RM calculator in your workout?

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We don’t shy away from tough work at Square One Active Recovery. We do what it takes to help you find true freedom from pain.

By now you may have sense that I used the 1RM calculator the reverse way. While most people would use it to estimate what is the maximum amount of weight they can lift in one repetition, I used to to estimate how many repetitions I can lift at a lighter weight.

The objective of doing this is so you can get an idea of what is an equivalent intensity workout at a lower weight or higher volume.

This is particularly useful if you feel like your technique is not as great at heavier weight. You want to take the weight down a notch to keep on your technique. At the same time, you also want to push yourself to a comparable intensity.

1RM calculator can serve as an objective guide but user discretion is required

If you are working your way upwards to do a true 1RM testing, please do it with a spotter. There is no room for error when you are doing a true 1RM. You either complete it or you fail. There is no in between.

When pushing for more reps, you have to option to stop before your target reps. So, there’s more wiggle room and, to some degree, it is safer for you if you are training alone.

When attempting to push to your limit, do take into consideration how you are feeling and performing that day. If you have just trained the same body part day before, it’s not a good idea to challenge yourself to a 1RM testing today.

If you don’t have enough sleep or if you are having a bad day (e.g., feeling frustrated), it’s also not a good idea to take it all out on  a 1RM testing.

We have our biases and sometimes we need a different perspective

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Sometimes what we need is not to try harder, but to have a coach to help us help ourselves get to our goals!

Specific to my case, I consistently underestimated how many reps I can do at a lower weight. Using an objective calculation, it allows me to push myself further than what I am comfortable with. In other words, a different perspective.

For some of you, you may be comfortable lifting lighter weights for many repetitions and think that you cannot lift heavier. With the 1RM calculator, you can get a sensing of how heavy can you lift at a lower rep range.

When in doubt, always seek professional help. Always remember that such tools are meant to supplement your own training and they are not 100% applicable to everyone. If you have been struggling with your workout or you are unsure of how to take your training to the next level, drop us a message to find out more about how we can help.


Frustrated by the lack of results-driven and ethical chiropractic clinics in Singapore, Chiropractor Jesse Cai found Square One Active Recovery to deliver meaningful and sustainable pain solutions.

Our goal? To make our own services redundant to you.

*We do not offer temporary pain relief such as chiropractic adjustments, dry needling, or any form of soft tissue therapy.