I need to make a decision on back pack size.

I am training up for a 85 mile hilly trail race in August here in the UK. It will be an over night event and I will need to carry my own safety equipment.

1. Hooded waterproof(wp) top
2. trousers (wp)
3. hat
4. gloves
5. maps and compass
6. head torch and hand torch
7. 3 pints fluid
8. Survival bag/space blanket
9. Spare shirt and socks
10. Food

This is not a full self-sufficient run as the there will be stocked check points along the way providing hot water and some food.

My question is 12 or 18 litre back pack?

Cheers Capster, my thinking too.

I have a 9 litre and it is jjjuuuusssttt that bit too small for the longer runs. I have sourced out a great pack that has a 12 litre capacity with external mesh if I need more.

Lighter is always better.  There is a strong correlation between longer finishing times and the amount of useless "just in case" crap that runners carry.  I have a 50 mile race this coming weekend and am planning to carry two handheld water bottles and a waistpack.  If your 9 litre pack suits you nicely, is there any way that you can make that work?  Do you need a spare shirt, for example?

 Spare shirt...no not really.

I am checking the rules for mandatory equipment and having done something similar it was just that little too tight even with a stuff sack. The survival bag is the thing that can take up the room so may just plump for the blanket