Reserve Capacity to Ah calculator
Convert a battery's Reserve Capacity (RC, in minutes) to amp hours (Ah) and back.
Handy when the label only prints RC and you want a rough capacity in amp hours.
Estimated capacity
41.7 Ah
Rated 20-hour Ah is usually a little higher than this - see the note below.
The formula, and its limit
Reserve Capacity is the minutes a battery holds a 25 amp load before dropping to 10.5 volts. Amp hours at that draw are RC x 25 / 60 (about RC x 0.4167), and RC is roughly Ah x 2.4. Treat it as an estimate: because 25 amps is a heavy draw, a battery's rated 20-hour Ah is usually a bit higher than this, especially for smaller batteries. When you need an exact figure, use the manufacturer's Ah rating.
Working from cold cranking amps instead? Use the CCA to Ah calculator →
Frequently asked
How do you convert Reserve Capacity to Ah?
Reserve Capacity is the number of minutes a 12V battery delivers 25 amps before dropping to 10.5 volts. To estimate amp hours, multiply RC by 25 and divide by 60 (RC x 0.4167). So an RC of 120 minutes is roughly 50 Ah.
How do you convert Ah to Reserve Capacity?
Multiply amp hours by about 2.4 to estimate Reserve Capacity in minutes. A 50 Ah battery is roughly 120 minutes of RC.
Why is my battery's rated Ah higher than this estimate?
RC is measured at a heavy 25 amp draw, and batteries deliver less total capacity the harder you pull from them (the Peukert effect). The manufacturer's 20-hour Ah rating uses a gentle draw, so it usually reads higher than the RC-based estimate, especially for smaller batteries.