The common rule of thumb is to dress for about 10°C warmer than the actual temperature, because running generates heat that takes 10 to 15 minutes to arrive. A morning that feels cold standing still usually calls for less clothing than instinct suggests. This tool applies that adjustment, then lists layers for the resulting dress-for temperature rather than the air temperature.
Throwaway layers, because the start-line wait is a different problem from the race. Tokyo starts at 9:10 a.m. with runners in blocks well before that, and its recent editions have had lows near 2°C. An old jumper, a bin bag or cheap gloves you can discard at the gun solve it without carrying anything for 42 kilometres. Most Majors collect and donate discarded clothing.
On the race-day weather in Marathon Ballot's data, Tokyo runs coldest on average, with recent starts between 7°C and 14°C and a 2°C low in 2024. London, New York and Boston sit close behind. At the other end, Berlin's 2025 edition started at 22°C and peaked near 28°C, and Sydney's 2023 heatwave pushed the high to 32°C. Every race in the picker uses its own most recent published race-day figure.
Rarely, once you are running. A jacket makes sense for driving rain or genuinely freezing conditions, and otherwise traps heat you will spend the second half trying to lose. A long-sleeve layer plus arm sleeves you can push down is more adjustable than a jacket you have to carry once it comes off.
Yes, and by more than most runners plan for. Published research on marathon finishing times shows pace slowing progressively above a fairly narrow optimal temperature band. The heat pace calculator applies that adjustment to a goal pace, which is worth checking alongside the kit list on any warm forecast.