Gutter and Downspout Calculator
Gutter feet, downspout and elbow counts from eaves.
What it does
Gutters fail at spacing before size. One downspout per 40 ft of eave keeps water moving inside the 50 ft practical max, so a 120 ft run needs 3 downspouts with 6 elbows. Longer runs without outlets overtop in cloudbursts regardless of profile.
The tool above counts parts from eave length. Enter feet of eave and it prints gutter feet, downspout count at one per 40 ft, and elbows at two each. Defaults show the 120 ft run; an 85 ft run also takes 3 while 38 ft takes 1.
Trade and federal sources agree. SMACNA caps gutter length per downspout at 50 ft with manual sizing tables behind, DOE Building America places downspouts every 20 to 50 ft with 1 sq in per 100 sq ft of roof, and Washington State Extension adds slope near 1/8 in per 10 ft with area-based downspout rules.
Slope and discharge finish the job. Quarter-bubble falls toward outlets prevent standing water and mosquitoes, and daylighting 5 ft or more from foundations protects basements. Big steep roofs may need 6-inch profiles by the area rule.
How to use
- Measure the eave length in feet.
- Read gutter feet first.
- Read downspout count.
- Read elbow count (2 each).
- Slope 1/16 in per ft to daylight.
- Discharge 5+ ft from foundation.
Formula
- eave
- Eave length needing gutter, in feet.
40 ft divisor sits inside DOE 20-50 ft range under the SMACNA 50 ft practical max. WSU adds area-based sizing for big roofs.
Worked example
Example: 120 ft eave
Defaults: 120 ft eave.
- Input: 120 ft of eave.
- Gutter: 120 ft of 5-inch K-style.
- Downspouts: ceil(120 / 40) = 3.
- Elbows: 3 x 2 = 6 (outlet + shoe each).
- Tool shows 120 ft gutter, 3 downspouts, 6 elbows.
Reference table
| Eave ft | Downspouts | Elbows |
|---|---|---|
| 30 | 1 | 2 |
| 40 | 1 | 2 |
| 41 | 2 | 4 |
| 80 | 2 | 4 |
| 120 | 3 | 6 |
| 200 | 5 | 10 |
Limits
- Length math only. Roof area can upsize profiles.
- One straight run assumed; hips and valleys differ.
- Snow and ice loads unmanaged.
- Underground drains need separate plans.
Common mistakes
- Spacing past 50 ft per downspout.
- Level gutters holding water.
- Daylighting at the foundation.
- Undersizing steep big roofs.
- Skipping elbows in the parts list.
Try it yourself
Quick checks. Answers sit tucked under each question.
85 ft eave?
85 / 40 = 2.125, ceil = 3 downspouts. Elbows 3 x 2 = 6.
38 ft garage eave?
38 / 40 = 0.95, ceil = 1 (minimum 1). Elbows 2.
Questions
How many downspouts?
One per 40 ft of gutter (50 ft max per SMACNA). Here ceil(120/40) = 3.
How many elbows?
Two per downspout: one offset at the outlet, one shoe at the base. Here 6.
What gutter size?
Five-inch K-style suits most homes; 6-inch for steep or large roofs per SMACNA sizing.
How much slope?
At least 1/16 in per foot toward downspouts; WSU notes ~1/8 in per 10 ft.
Where should they discharge?
Five feet or more from the foundation, 10 ft to catchment, per DOE guides.
Is length the only sizing?
No. Area rules (1 sq in per 100 sq ft roof) can demand bigger profiles on large roofs.
Sources
- SMACNA: Gutter Sizing Notes (50 ft max per downspout)
- DOE Building America: Gutters and Downspouts (20-50 ft)
- WSU Extension: Managing Roof Runoff