"g per £1" is the total weight, in grams, of that denomination needed to make up £1 of value — for a single coin or note this equals its individual weight divided by its face value. It's the only number the calculator actually needs; face value is kept for reference and sorting.
From the ratios you supplied (weight ÷ value) for coins from 1p to £2, and from the £5–£50 note samples (e.g. 100 g of notional value in £10s weighing 83 g). Edit any figure below — everything recalculates immediately, including entries already on the Calculator tab.
No counts saved yet. Use "Save to history" on the Calculator tab after entering a count.