Calculator
AGM to Lithium Battery Size Calculator
Your existing AGM capacity, returning the lithium bank that delivers the same usable energy and the weight you take off the boat.
The nameplate total across the whole house bank.
AGM gives you about half its nameplate before the chemistry objects; LiFePO4 gives you 80%. That single difference is why the lithium answer is always the smaller, lighter number.
Your numbers
400 Ah
Lithium to replace 600 Ah of AGM
| Usable from the AGM bank | 300 Ah |
| Lithium bank required | 400 Ah |
| Usable from the lithium bank | 320 Ah |
| AGM weight | 179 kg |
| Lithium weight | 50 kg |
| Weight off the boat | 128 kg |
Calculator assumptions and limits
- AGM usable capacity: 50% of nameplate
- LiFePO4 usable capacity: 80% of nameplate
- Pack weights: roughly 8 kg per 100 Ah at 12 V for LiFePO4, 28 kg for AGM
Every assumption here is versioned and dated on the methodology page. If you disagree with one, the working is there to argue with.
Battery sizing calculation guides
The same arithmetic this calculator runs, written out step by step.
Is a Lithium Boat Battery Upgrade Worth It? Decision GuideLithium pays for itself where you cycle the bank often and deeply — at anchor, on a liveaboard, or with a fridge running year-round. It rarely pays on a boat that sits on shore power and goes out for a day at a time.How to Convert a Boat from AGM to Lithium BatteriesSize the new bank on usable capacity rather than matching the old label figure, fix the charging path before connecting anything, and replace the fusing. A straight case-for-case swap is the most common way to make this go wrong.
The other calculators
House bank calculatorYour loads, your nights away from shore power, your solar and your alternator, returning the bank, the DC-DC charger, the AGM equivalent and the weight off the boat.Trolling motor runtimeThrust rating, the speed you actually troll at and your bank size, returning hours on the water and the bank you need for a full day.DC-DC and alternator sizingAlternator rating and bank capacity, returning the charger to fit and the time to refill from your engine.Solar top-upPanel watts against your daily draw, returning the average daily return and whether the array covers your consumption.