Plan a picnic menu and quantity.
Picnic food scales with people, but drinks scale with people AND time AND heat, which is why they are calculated separately — four hours in 30 C weather needs roughly double the water of a mild afternoon. Ice is sized from the drink volume it has to chill plus an allowance for heat soaking into the box, since a cool box in direct sun loses ice far faster than the drinks alone would suggest.
Food
grams = adults x 500 + children x 300
Drinks
litres = people x ml per person-hour x hours / 1000
Ice
kg = litres x 0.5 (+ 0.5 per hour in hot weather)
Perishable picnic food should not sit in the 5-60 C range for more than two hours, or one hour above 30 C. Discard anything left warm for longer.
Sweat losses can exceed half a litre an hour in heat. The 400 ml per person-hour figure keeps people ahead of thirst rather than catching up, which is when headaches and fatigue set in.
Keep chilled food under 5 C and aim to serve within two hours of leaving the fridge — one hour if it is above 30 C. Pack raw and ready-to-eat items separately.