How to use this calculator
- 1Measure every roof edge that will get a gutter and add the lengths together.
- 2Count the runs — each separate gutter with two ends — and any corners where a run turns around the house.
- 3Choose the gutter length you will buy. Keep the downspout and hanger spacing unless your product says otherwise, and choose heavy snow if you get it.
- 4Optional: enter the height from the ground to the gutter to get the length of downspout pipe.
The formula
- 01Straight stretches = runs + corners; each stretch = total length ÷ stretches
- 02Sections: each stretch is made of whole lengths plus one shorter piece; short pieces are cut from one length where they fit
- 03Connectors = joints inside each stretch (lengths per stretch − 1)
- 04Downspouts = runs × (length of each run ÷ most gutter per downspout, rounded up)
- 05Hangers = stretches × (stretch ÷ spacing, rounded up, + 1)
- 06End caps = 2 × runs; elbows = 3 × downspouts
A run is a continuous gutter with an end cap at each end. Where it turns a corner, a corner piece (a mitre or angle) joins two straight stretches. Inside each stretch the lengths are joined with connectors, also called seamers, slip joints or unions.
Amerimax's planning guide uses one outlet (downspout) per 40 ft of gutter, one hanger every 24 in (every 18 in in heavy ice and snow areas) and three elbows per downspout. FloPlast allows brackets up to 1 m apart on its standard PVC gutters — closer for some systems, and 400 mm where heavy snow is likely — with a bracket within 150 mm of every angle and stop end. So the calculator adds a hanger at each end of every stretch. The downspout rule is a simple guide; large or steep roofs and heavy rainfall may need more, so check the manufacturer's flow tables.
Section counts assume you don't join short offcuts together. That keeps the joints to a minimum and never leaves you short.
Worked example
80 ft of gutter in 2 runs, bought in 10 ft sections, needs 8 sections joined with 6 connectors, plus 4 end caps.
At one downspout per 40 ft you need 2 downspouts with 6 elbows and 20 ft of pipe, and at 24 in spacing, 42 hangers.
Tips before you buy
- Put downspouts where the water can drain safely away from the house, and add an extension or splash block to carry it clear of the foundation.
- Place outlets at the low end of each run, or in the middle of a long run so the water flows both ways.
- Buy gutter sealant for aluminium gutters and silicone lubricant for PVC seals — both are easy to forget.
- Leave the expansion gap marked on PVC gutters: FloPlast allows for about 10 mm of movement per 4 m length.
- Fit leaf guards or outlet strainers if trees overhang the roof.
Frequently asked questions
How many downspouts do I need?
At least one per run, and Amerimax plans one outlet per 40 ft of gutter. Large, steep roofs or areas with heavy downpours may need more; the gutter maker's flow tables give the exact figure for your roof area.
How far apart should gutter hangers be?
Amerimax asks for one hanger every 24 in, and every 18 in in heavy ice and snow areas. FloPlast allows up to 1 m on standard PVC gutters and 400 mm where heavy snow is likely. Always follow your gutter system's instructions.
How much slope should a gutter have?
Only a slight fall toward the outlet: about ¼ in per 10 ft according to Amerimax, or 1:350 (roughly 3 mm per metre) according to FloPlast.
What size gutter do I need?
5 in K-style gutters in the US and 112 mm half-round gutters in the UK are common on houses, but the right size depends on the roof area, its pitch and local rainfall. Check the manufacturer's flow tables, or BS EN 12056-3 for the full calculation.
What is the difference between a gutter run and a section?
A section is one length you buy, such as 10 ft, 3 m or 4 m. A run is the whole gutter between two end caps, made of several sections joined with connectors.
Do seamless gutters need connectors?
Seamless gutters are formed on site in one piece per run, so there are no joints between sections — only corners. Use the length, downspout, hanger and end cap results and ignore the section and connector counts.
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Sources
- Amerimax Metal Gutter System planning and installation sheet (1 outlet per 40 ft, hangers every 24 in / 18 in, 3 elbows per downspout)
- FloPlast PVC-U rainwater installation guide (bracket spacing, snow areas, 1:350 fall)
- FloPlast rainwater systems technical data (thermal movement, brackets near angles and stop ends)
Formulas last reviewed: September 30, 2026 ·How we check our formulas