The formula
nappies = sum of each month's daily changes × 30.44 days
Where it comes from
The number of changes is not constant: a newborn goes through more than ten a day and a two-year-old fewer than five, so multiplying by an average gives a number that matches reality at no point at all. The calculation runs month by month at the rate for each age, which is what explains why the first year takes almost half the total.
How to work it out by hand
- Look up how many changes a day belong to each age band
- Multiply by 30.44, the days in an average month
- Add up the months left until they are out of nappies
- Multiply by the unit price
What is worth knowing
The price per unit is where the spending is really decided, and it varies more than you would think: own-brand and premium can differ by a factor of two, and buying in large boxes usually cuts the unit price by twenty or thirty per cent. It is worth working out the price per nappy by dividing the pack by the count, because pack sizes change and the price on the shelf is not comparable. And the age of potty training matters more than anything else: six months either way is about eight hundred nappies.