Interactive Calculator · 2026
Gutter Size Calculator
Size a rainwater gutter from the roof it drains. Enter the roof plan area and the rainfall intensity — get the peak gutter flow in litres per second, a recommended nominal half-round gutter width and the volume of water it must carry in one hour of storm.
Gutter flow rises with roof area (at 75 mm/hr rainfall)
Your roof & rainfall
Plan (footprint) area of roof feeding this gutter.
Computed from area × rainfall (runoff C ≈ 1).
Design storm intensity for your city — most Indian towns use a 5-minute peak of roughly 75–150 mm/hr.
Gutter flow across roof areas
Larger roofs push more water into the same gutter — the width has to keep pace.
At 75 mm/hr your 50 m² roof sheds about 1.04 L/s, which falls in the band for a nominal 100 mm half-round gutter.
Gutter capacity also depends on the fall, the section shape and how far apart the outlets are — these are indicative bands, so verify against manufacturer tables. Closer outlet spacing lets a smaller gutter cope with the same roof.
How this is calculated
- Gutter flow = intensity × area ÷ 3600 (runoff coefficient C ≈ 1 for a roof) = 75 × 50 ÷ 3600 = 1.04 L/s.
- Recommended width = nominal half-round band for 1.04 L/s = 100 mm (bands: ≤1.5 → 100, ≤3 → 125, ≤5 → 150, ≤8 → 200, else 250 mm — indicative).
- Peak volume in 1 hr = gutter flow × 3.6 = 1.04 × 3.6 = 3.75 m³.
Indicative sizing for concept planning. A detailed design must fix the local design storm, the gutter fall and section, and the outlet layout — confirm with a qualified consultant, manufacturer data and NBC 2016 Part 9 before procurement.
Frequently asked questions
- How does the gutter size calculator work?
- It sizes a rainwater gutter using the rational method for a roof. Peak gutter flow in litres per second equals rainfall intensity in millimetres per hour multiplied by the roof plan area in square metres, divided by 3600, taking a runoff coefficient of about 1 for a hard roof. That flow is then matched to an indicative nominal half-round gutter width band.
- What inputs do I need and what values should I use?
- You need two numbers: the roof plan area drained by the gutter in square metres, and the design rainfall intensity in millimetres per hour. Use the footprint area of the roof feeding that one gutter. For intensity, most Indian towns use a short-duration peak of roughly 75 to 150 mm/hr; check your city's design storm, as coastal and heavy-monsoon areas run higher.
- How accurate is this gutter size and what should I verify?
- The result is indicative for concept planning only. Real gutter capacity also depends on the fall, the section shape and how far apart the outlets are, so closer outlets let a smaller gutter carry the same roof. Treat the width bands as a starting point and confirm against manufacturer capacity tables, a qualified consultant and NBC 2016 Part 9 before you procure.
Related Guides — Deep-dive reading
Stormwater & Surface Drainage in India: Getting Rain Off Your Roof, Paving and Plot
A homeowner's guide to stormwater drainage — sizing roof outlets and downpipes for monsoon rainfall, surface channels and catch basins, grading the plot away from the house, and where the water finally goes, kept firmly separate from your foul drains.
PlumbingRooftop Rainwater Harvesting in India: Catching Rain Off Your Roof for Storage or Recharge
A homeowner's guide to rooftop rainwater harvesting — using the roof as a catchment, sizing gutters and downpipes for your area and rainfall, first-flush and filtration, and where the water finally goes: a storage sump for use or a recharge structure for the ground.
PlumbingRainwater Recharge Pit for Indian Homes: How It Works, Sizing, Filter Media, Distances and Maintenance
The soak-pit that puts rain back into the ground — how a rainwater recharge pit works, how to size it against your roof area and soil percolation, the boulder-gravel-sand filter media, the desilting chamber ahead of it, safe distances from foundation, borewell and septic, why it fails in clay, and how to keep it from choking.
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