First look for Wh on the battery
For air travel, the battery's energy rating is what matters. If Wh is printed, use that value rather than recalculating it.
If Wh is missing
Calculate Wh = Ah × V, or mAh × V ÷ 1000. Keep the manufacturer's specification page or product documentation with you.
A handwritten label is not magic
Adding your own calculated Wh note can make inspection easier, but it does not transform an undocumented battery into a manufacturer-certified item. Some airlines explicitly require clear power-rating information on the item.
Custom FPV packs
For custom or hobby packs, keep the original pack label intact and carry documentation that shows nominal voltage and capacity. The live checker lets you mark whether clear rating information is available and will warn when a selected airline is known to require it.