The magic number to remember is 4. 

4ml/kg of packed red blood cells will give you an increase of about 1g/dL of haemoglobin. 

For example a 9 kg child with an HB of 5g/dL will need 180ml of red cells to reach an HB of 10. (9kg X 4ml X 5g/L = 180ml)

PS - You can use this formula for adults too. 

An easy way to estimate the weight of a child without a formula:

- Term baby 3kg

- 1 year old 10kg

- 5 year old 20kg

Formulae can be hard to remember, especially in paediatrics where most of the terms of the various formulae are similar: Is it age/4+4, or 2(age+4), or 12 + (age/2)? The weight formula 2(age+4)kg is also becoming less accurate as the population becomes heavier and heavier.