How it works
Your max is estimated with the Epley formula: weight × (1 + reps ÷ 30). Squat 225 lb for 5 reps and your estimated one-rep max is about 265 lb. Numbers are rounded to the nearest 5 lb (or 2.5 kg) so they match the plates on the bar.
Getting the most accurate number
Use a set you took close to failure, with no more than a rep or so left in the tank, and clean form. Sets of 3 to 6 reps give the closest estimate. Past about 10 reps the guess drifts, because high-rep sets lean more on endurance than strength. Some people also grind out more reps at a given weight than others, so treat it as a strong estimate, not a promise.
How to use your max in training
Most programs work off percentages. Strength work usually sits around 80–90% of your max for 3 to 6 reps. Building muscle usually sits around 65–80% for 6 to 12 reps. Lighter percentages are for warm-ups and technique. Re-check your number every 4 to 8 weeks from a hard working set.
Do you need to test a true max?
No. You can train well for years off estimates like this one. If you do test a real single, warm up well, use a spotter or safety pins, and stop the moment your form breaks down.