Gutter Downspout Calculator
Downspout count and spacing for any gutter run.
What it does
Gutters fail at the ends and middles when downspouts sit too far apart. A common planning rule allows about 40 ft of gutter per downspout with at least two per run, then spacing splits the run evenly. This page runs that rule in feet or meters.
The default is a 120 ft run. Division gives exactly 3 downspouts at 40 ft apart. Metric defaults convert the same run to 36.6 m and give the same 3 at 12.2 m apart, since the metric divisor is the exact 40 ft.
Corners and valleys override even math. Each outside corner wants a downspout, and a valley dumps two slopes into one bay that needs its own drop. Treat the tool count as a minimum and walk the roof edge before buying.
Count is not capacity. Pipe size, elbows, slope to the drop, and where water goes at the ground decide whether the run drains a cloudburst. Size those with local guidance for big roofs.
How to use
- Pick imperial or metric first.
- Enter the gutter run length.
- Read count first (at least 2).
- Read spacing next.
- Move spots to corners and valleys on site.
Formula
- length
- Straight gutter run length.
- 40 ft
- Max ft per downspout for plain runs.
- minimum 2
- Floor of two per run, even short ones.
The 40 ft rule is a planning spacing for plain runs; corners need one each and valleys feed extra water to one point. Metric divides by 12.2 m, the exact 40 ft, so both modes agree. Large or steep roofs need a drainage check beyond count.
Worked example
Example: 120 ft run
120 ft gutter (metric 36.6 m).
- Count: 120 / 40 = 3 downspouts.
- Check minimum: 3 beats the 2 minimum.
- Spacing: 120 / 3 = 40 ft apart.
- Metric: 36.6 m gives 3 at 12.2 m apart.
- Tool shows 3 downspouts imperial and metric.
Reference table
| Run | Count | Spacing |
|---|---|---|
| 40 ft | 2 | 20 ft |
| 80 ft | 2 | 40 ft |
| 120 ft | 3 | 40 ft |
| 160 ft | 4 | 40 ft |
| 200 ft | 5 | 40 ft |
Limits
- Planning count only. No pipe sizing.
- Ignores roof area and pitch beyond the note.
- Even spacing assumed. Corners shift spots.
- Drainage at grade sits outside it.
Common mistakes
- Running one long line with a single drop.
- Spacing by eye across corners.
- Mixing feet with meters in length.
- Forgetting valleys double feed one bay.
- Pitching gutters away from the drop.
Try it yourself
Quick checks. Answers sit tucked under each question.
80 ft run?
2 downspouts (80 / 40 = 2), 40 ft apart.
30 ft run?
2 downspouts (minimum), 15 ft apart.
Questions
How many downspouts?
Length over 40 ft, at least 2. Here 120 ft needs 3.
How far apart?
Length over count. Here 40 ft apart.
What is that in metric?
3 at 12.2 m apart for 36.6 m.
When do I add more?
Steep or large roofs shed faster and want more than the length rule.
Does it size the pipe?
No. Size, elbows, and drains sit outside count math.
Is spacing even?
Yes for planning, but corners and valleys often set real spots.