Gutter and Downspout Calculator
Works out gutter length, brackets, fittings and how many downspouts a roof needs. Downspouts are sized on the area draining into them, not on the length of gutter.
Includes 5% for cutting.
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The formula
gutter = eaves length × 1.05 downspouts = ceiling(roof area ÷ area each drains) brackets = ceiling(eaves ÷ spacing) + 1
Downspout count is driven by roof area rather than gutter length, because it is the volume of water arriving that matters. A long gutter draining a shallow roof may need one outlet; a short gutter under a large steep roof may need two.
What to watch out for
The parts of this that are not just measuring:
- Downspouts follow area, not length. A standard 110 mm round pipe handles roughly 50 m² of roof in temperate rainfall. Two short runs may need two outlets; one long run may need only one.
- Fall of about 1:350. Around 3 mm per metre towards the outlet. Too little and it pools; too much and it looks wrong against a level fascia.
- Bracket spacing is about load, not span. 800 mm to 1 m normally, closer under a roof that sheds snow — a gutter full of ice and sliding snow is what pulls brackets off a fascia.
The capacity figures here are rules of thumb for temperate rainfall. Regions with intense downpours size gutters from rainfall intensity data, and the local standard governs.
Frequently Asked Questions
How many downspouts do I need?
About one per 50 m² of roof for a standard 110 mm round downpipe. A 100 m² roof needs two. High-rainfall areas need more capacity and should be sized against local rainfall data.
What fall should a gutter have?
Roughly 1:350, or 3 mm per metre, towards the outlet. That is enough to drain without standing water and shallow enough that the gutter still reads as level.
How far apart should gutter brackets be?
800 mm to 1 m for most systems. Reduce to 600 mm where snow slides off the roof — the weight of snow and ice in a gutter is what tears brackets out of a fascia.