Size gutters and downspouts based on roof area and rainfall
Our rain gutter sizing calculator determines the optimal gutter and downspout sizes based on your roof area, pitch, and local rainfall intensity. Properly sized gutters prevent overflow, foundation damage, and erosion while ensuring efficient water management.
Undersized gutters overflow during heavy rain, causing water damage and defeating the purpose of the gutter system. The calculator accounts for roof pitch (which increases water velocity) and rainfall rates to recommend appropriate gutter capacity and downspout quantity.
This calculator helps contractors, DIYers replacing gutters, and anyone asking "what size gutters do I need?" or "how many downspouts for my roof?" Local rainfall intensity data is available from NOAA or local building departments.
Follow these steps to size your gutters: