Gutter Calculator
Work out how much guttering a roof needs — total length and the number of downspouts, from the eave length and spacing.
Gutters are sold by the foot, and the count that matters is the downspouts — too few and the gutter overflows in heavy rain. This calculator gives you both numbers from the eave length.
Measure every run, add them up, and set the spacing to your roof's needs.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure every eave run (the gutter line) and add the lengths.
- Enter the downspout spacing — 40 ft is a good default.
- Add a price per foot or metre if you want a cost estimate.
Worked example
| Gutter length | 120 ft |
|---|---|
| Downspout spacing | 40 ft |
| Downspouts | ⌈120 ÷ 40⌉ = 3 downspouts |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
How many downspouts do I need?
One per 30–40 ft of gutter, and at least one per run. A 120 ft gutter needs 3 downspouts at 40 ft spacing. Long runs and valleys need more — water must not travel far to a downspout.
What size gutter do I need?
Standard 5 in (125 mm) gutters suit most houses; 6 in (150 mm) suits large or steep roofs that shed water fast. This calculator gives the length — size it with your supplier for your roof area and rainfall.
How do I measure gutter length?
Measure along each eave from end to end, including corners, and add the runs together. Do not deduct for downspouts — the gutter runs past them.
Do gutters need a fall (slope)?
Yes — about 1 in 40 (roughly ¼ in per 10 ft) towards the downspout, so water flows instead of pooling. Set the fall when you install, not when you order.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.