How to use the gutter size calculator
A gutter that is too small overflows at the very moment it matters, and the water ends up against the facade, in the window heads and at the foundations. Sizing one properly is a short calculation: roof area times rainfall gives a flow, and the flow picks the gutter and downspout. This calculator follows the method in Chapter 11 of the International Plumbing Code (IPC 2021), which sizes gutters and leaders by flow in gallons per minute.
- Enter the roof area that drains to this gutter, measured on plan, not along the slope. For a simple gable that is the eave length times the horizontal distance from eave to ridge.
- Add any wall area that sheds rain onto that roof, such as a taller wall above a lean-to. Half of it is added to the drainage area (IPC 1106.4).
- Enter the design rainfall rate. In the US that is the 100-year, 1-hour rate from the IPC maps or local data; elsewhere use your local design intensity.
- Enter how many downspouts share the flow, assuming outlets are placed so each drains an equal part of the gutter.
- Choose the gutter slope you can achieve, then read the flow and the smallest gutter and downspout sizes that carry it.
Gutter sizing formulas
Drainage area = roof area on plan + ½ × wall area shedding onto the roof
Flow (gpm) = area (ft²) × rainfall (in/h) × 0.0104 · Flow (L/s) = area (m²) × rainfall (mm/h) ÷ 3,600
Flow per downspout = flow ÷ number of downspouts
Gutter: capacity in IPC Table 1106.6 ≥ flow per downspout
Leader: capacity in IPC Table 1106.3 ≥ flow per downspout
The 0.0104 constant is 1 in of rain per hour over 1 ft² (0.0833 ft³ an hour) converted to gallons per minute. One litre per second is about 15.85 gpm.
| Half-round gutter (IPC Table 1106.6) | Slope | Capacity |
|---|---|---|
| 4 in | 1/8 in/ft | 39 gpm |
| 5 in | 1/8 in/ft | 74 gpm |
| 6 in | 1/8 in/ft | 110 gpm |
| 8 in | 1/16 · 1/8 in/ft | 172 · 247 gpm |
| 10 in | 1/16 · 1/8 in/ft | 331 · 472 gpm |
The table also lists rectangular gutters from 1½ × 2½ in to 4 × 10 in at steeper slopes, and the calculator checks those too. Round leaders in Table 1106.3 run from 2 in (30 gpm) through 2½ in (54), 3 in (92), 4 in (192), 5 in (360) and 6 in (563) to 8 in (1,208 gpm).
Worked examples
A small gable roof
One side of a roof 40 ft long and 15 ft from eave to ridge on plan drains 600 ft². At 3 in/h the flow is 600 × 3 × 0.0104 = 18.7 gpm. A 4 in half-round gutter at 1/8 in per foot (39 gpm) carries it to one 2 in round leader (30 gpm). A 2 × 3 in downspout rated conservatively as 2 × 2 in (30 gpm) also works.
A larger roof with a wall above it
A 1,200 ft² roof below a 200 ft² wall drains 1,200 + 100 = 1,300 ft². At 4 in/h that is 54.1 gpm. With one outlet the gutter needs to be a 5 in half-round (74 gpm), and the leader a 3 in round (92 gpm), because a 2½ in leader is rated at 54 gpm, just short. With two downspouts each takes 27 gpm, and a 4 in half-round gutter with 2 in leaders is enough.
A metric example
A 150 m² roof at a design intensity of 75 mm/h gives 150 × 75 ÷ 3,600 = 3.1 L/s, or about 49.5 gpm. On one outlet, the smallest half-round gutter at 1/8 in per foot is 5 in (127 mm, 74 gpm), and the smallest round leader is 2½ in (54 gpm). Check the result against the method in your local code, which may differ.
Who this calculator helps
- Architects and technologists sizing rainwater goods and checking downspout positions on elevations.
- Roofing and gutter contractors backing up a quote with a quick code check.
- Plumbing engineers doing an early check before the full storm drainage design.
- Building inspectors and plan reviewers verifying gutter and leader sizes on drawings.
- Homeowners wondering why their gutters overflow in a heavy storm.
Practical tips
- Put downspouts near the far ends and at valleys, where the most water arrives, and avoid long runs that drain to a single outlet.
- Measure roof areas from the plan with the square footage calculator, and get the roof slope from the roof pitch calculator.
- Pair this with the roofing calculator to take off the roof and the rainwater goods together.
- Leave room for leaf guards and debris: a gutter at its full rated capacity has no margin once it starts to clog.
- Discharge downspouts well away from foundations, or into a drain or soakaway your local authority accepts.
Want to show a client new gutters and colours on their house? Try the change on a photo of the facade with Exterior Redesign.
