Gutter Size Calculator

Example data table

Examples are illustrative; confirm your project requirements and local rules.

Formula used

• Design area: A_d = A_plan × F_pitch
• Runoff flow: Q = A_d × I × 0.0103896 (gpm)
• Per-downspout: Q_ds = Q ÷ N
• Per-foot demand: q = Q_ds ÷ (L ÷ N)
• Safety: q_req = q × (1 + S/100)

Where A is in ft^2, I is rainfall intensity in inches per hour, L is total gutter length in feet, N is downspout count, and S is the safety factor percent.

How to use this calculator

1. Select your unit system, then choose area input method.
2. Enter plan area, or enter roof length and width.
3. Pick a pitch factor matching your roof slope range.
4. Enter local rainfall intensity for your design storm.
5. Enter the gutter length that drains the roof area.
6. Enter how many downspouts serve that gutter run.
7. Choose gutter profile and set a safety factor.
8. Press Calculate, then download CSV or PDF if needed.

Professional field guide to gutter sizing

1) Start with the contributing roof area

Use the plan (horizontal) roof area as your baseline. If your roof has multiple planes, sum the sections that drain into the same gutter run. When you only know dimensions, length multiplied by width gives plan area. Larger areas create higher runoff and demand bigger gutters.

2) Apply a pitch factor for real collection

Steeper roofs shed water faster and can push more flow toward the eave. A practical approach is multiplying plan area by a pitch factor, commonly 1.00 to 1.30. This calculator uses that factor to create a design area, helping you avoid undersizing on steep slopes.

3) Use local rainfall intensity, not averages

Gutters should be sized for short, intense storms rather than monthly or annual averages. Enter intensity as inches per hour (or millimeters per hour). A change from 4 to 8 in/hr doubles the runoff rate. If your region has published design storms, use those values.

4) Convert area and intensity into flow

Runoff flow is estimated with Q = A × I × 0.0103896, producing gallons per minute when area is in square feet and intensity is in inches per hour. The constant reflects unit conversion and a conservative runoff assumption for typical roof surfaces.

5) Distribute flow across downspouts and run length

Total flow is split by the number of downspouts serving the run. The calculator also estimates the gutter length per downspout, then computes a per‑foot demand. Longer runs feeding one outlet raise the required capacity, so adding an outlet can be as effective as increasing gutter size.

6) Add a safety margin for debris and concentration

A safety factor (0–50%) accounts for leaves, partial blockages, splash‑over, and valleys that concentrate water. Many projects use 10–25% in wooded areas or where maintenance access is limited. Safety margin is applied directly to the per‑foot capacity requirement.

7) Choose profile and verify capacity

K-style gutters typically carry more water than half-round at the same nominal size. This calculator compares the required per‑foot demand to approximate capacity ranges and recommends a nominal size. If results exceed standard sizes, consider larger custom profiles or additional outlets.

8) Detail for durability, overflow, and compliance

After sizing, confirm bracket spacing, fascia strength, slope, and outlet placement. Provide overflow paths away from foundations, especially near doors and walkways. Material choice (aluminum, steel, or copper) and seam quality affect long‑term performance, while local codes may set minimum requirements.

FAQs

1) What rainfall intensity should I enter?

Use a local design-storm intensity for short durations (often 5–15 minutes). If you only have hourly intensity, use the conservative higher value for your area to reduce overflow risk.

2) Why does roof pitch change gutter sizing?

Pitch affects how quickly water reaches the eave and how concentrated flow becomes. A pitch factor increases the effective collection area, helping size gutters for steeper roofs that shed faster.

3) Should I size for the entire roof or one gutter run?

Size each run based on the roof area that drains into it. A large roof with multiple downspouts often needs smaller gutters per run than a single long run serving the same total area.

4) How do valleys affect the result?

Valleys concentrate water into one section of gutter. If a valley feeds a specific run, allocate that contributing area to the run and consider a higher safety factor or extra outlets near the valley.

5) Is half-round always smaller capacity than K-style?

Often yes at the same nominal size because of shape and cross-sectional differences. Half-round can still work well, but may require stepping up a size or adding downspouts for equivalent performance.

6) What safety factor is reasonable?

Use 10–15% for clean roofs with routine maintenance. Use 20–30% for trees, screens, or limited access. Higher margins help, but proper downspout layout usually provides the biggest improvement.

7) Do these results replace engineering or code checks?

No. Treat the output as planning guidance. Verify with manufacturer data, local regulations, overflow requirements, and site-specific conditions such as wind-driven rain, snow, and drainage routing.