How to read the result
The drop is how far the outlet end sits below the high end. Fix the high end first, then measure down by the drop at the outlet and snap a line between them.
- Single outlet: the whole gutter is one drainage run, so the full length sets the drop.
- High at center: each half drains to its own outlet, so each run is half the length and the drop is half as large. A 32 ft gutter at 1/4 in per 10 ft falls 0.40 in to each end instead of 0.80 in.
- Marking points: the table under the calculator gives each mark's distance from the left end and how far it sits below the high point.
- Check mode: enter the fall you measured over one run to see the pitch it gives, in every unit.
This tool converts geometry only. The right pitch depends on your gutter product. Use the value in the manufacturer's installation instructions.
Worked example
A 32 ft gutter drains to one downspout at the right end. Suppose the product instructions call for 1/4 in per 10 ft. How far does the outlet sit below the high end?
- Pitch per foot = 1/4 in ÷ 10 ft = 0.025 in/ft
- Drop = drainage-run length × pitch = 32 ft × 0.025 in/ft = 0.80 in
- Grade = 0.025 in/ft ÷ 12 in/ft × 100 = 0.208%, a ratio of 1 : 480.0
- A mark 10 ft from the high end sits 10 × 0.025 = 0.250 in below it
These are the same numbers the calculator shows when it first loads.
Method and formulas
Drop = Lrun × p
Lrun = L (one outlet) or L ÷ 2 (high at center)
Offset at a mark = (distance from the high point) × p
Grade (%) = p ÷ 12 × 100 · Angle = arctan(p ÷ 12)
1 in = 25.4 mm; 1 ft = 0.3048 m (exact)
- L
- Total gutter length, measured horizontally (ft or m)
- Lrun
- Length of one drainage run, from the high point to an outlet
- p
- Pitch, in inches of fall per foot of run (1/4 in per 10 ft = 0.025 in/ft)
Assumptions and limits
- Straight gutter run with a uniform pitch.
- Pitch is chosen by the user from the product instructions.
- The calculator does not size gutters or downspouts and does not check any code requirement.
- The marking list stops at 501 rows. On very long runs it thins the marks evenly and always keeps both ends and the high point.
Reference tables
| Gutter length | 1/8 in per 10 ft | 1/4 in per 10 ft | 1/2 in per 10 ft |
|---|---|---|---|
| 10 ft | 0.125 in | 0.250 in | 0.500 in |
| 20 ft | 0.250 in | 0.500 in | 1.000 in |
| 30 ft | 0.375 in | 0.750 in | 1.500 in |
| 40 ft | 0.500 in | 1.000 in | 2.000 in |
| 50 ft | 0.625 in | 1.250 in | 2.500 in |
Source: Calculated with the EngiFormula gutter slope engine (method v1.1). Unit conversion uses the exact definitions 1 in = 25.4 mm and 1 ft = 0.3048 m, NIST Special Publication 811 (2008 edition), Appendix B.
The same pitches in other units
| Pitch | Per foot | Per metre | Grade | Ratio |
|---|---|---|---|---|
| 1/8 in per 10 ft | 0.0125 in/ft | 1.042 mm/m | 0.104% | 1 : 960 |
| 1/4 in per 10 ft | 0.025 in/ft | 2.083 mm/m | 0.208% | 1 : 480 |
| 1/2 in per 10 ft | 0.05 in/ft | 4.167 mm/m | 0.417% | 1 : 240 |
Source: Calculated with the EngiFormula gutter slope engine (method v1.1). Unit conversion uses the exact definitions 1 in = 25.4 mm and 1 ft = 0.3048 m, NIST Special Publication 811 (2008 edition), Appendix B.
Example pitches from manufacturer instructions
| Gutter type | Stated pitch | Document |
|---|---|---|
| Metal (aluminum or steel) K-style gutter | About 1/4 in per 10 ft | Installing Metal Gutter Systems (Amerimax Home Products (via Lowe’s product documents), undated PDF, retrieved 2026-10-10) |
| Traditional vinyl gutter | 1/8 in per 10 ft | Installing Traditional Vinyl Gutter Systems (Amerimax Home Products (via Lowe’s product documents), undated PDF, retrieved 2026-10-10) |
Source: Values as stated in the documents listed under Sources. Other products may call for a different pitch; follow your own product's current instructions.
Common uses
- New K-style gutter installation: find the outlet drop and mark the fascia at even intervals.
- Checking a sagging gutter: measure the fall over the run and see what pitch it gives.
- Long runs with two downspouts: set the high point at the center and halve each run.
Frequently asked questions
What is the correct slope for gutters?
It depends on the product, so use the pitch in the instructions for the gutter you are installing. For example, the guide Installing Metal Gutter Systems says to lower the line about 1/4 in for every 10 ft, and Installing Traditional Vinyl Gutter Systems states 1/8 in per 10 ft.
What is the standard pitch for gutters per 10 feet?
There is no single figure that applies to every gutter. The two guides listed under Sources give about 1/4 in per 10 ft (metal) and 1/8 in per 10 ft (traditional vinyl). The calculator works with any pitch you enter.
What is the ideal slope ratio for gutters?
The ratio follows from the pitch your product calls for. About 1/4 in per 10 ft is a ratio of 1 : 480.0, and 1/8 in per 10 ft is 1 : 960.0: the gutter falls 1 unit for every 480 or 960 units of run.
What is 1/4 inch per 10 feet as a percentage or in degrees?
It is 0.208%, or 0.119°. That is also 0.025 in per foot and a ratio of 1 : 480.0.
How much should a gutter fall per metre?
1/4 in per 10 ft is 2.083 mm per metre and 1/8 in per 10 ft is 1.042 mm per metre. Switch the calculator to metric to work in millimetres; it converts the inch figure exactly rather than using a rounded metric value.
How do I mark gutter slope on the fascia?
Mark the high point, then measure down by the offset at each marking point in the table and snap a chalk line through the marks. With a 2 ft interval at 1/4 in per 10 ft, each mark sits 0.050 in lower than the one before.
How many downspouts do I need for 40 feet of gutters?
This calculator does not size gutters or downspouts. The number and size of downspouts depend on the roof area, the design rainfall and the gutter size, so use a sizing method such as the one in SMACNA's Architectural Sheet Metal Manual or your gutter maker's guidance.
Sources and references
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), 2008 edition, Appendix B (exact values for the inch and the foot).
- Drop, grade, ratio and angle relations (method GUT-GEO-001): straight-line geometry, as set out in the Method section above. Unit conversion (GUT-UNIT-001): exact definitions from NIST SP 811.
- Amerimax Home Products (via Lowe’s product documents), Installing Metal Gutter Systems, undated PDF, retrieved 2026-10-10. It gives about 1/4 in per 10 ft as an approximate figure.
- Amerimax Home Products (via Lowe’s product documents), Installing Traditional Vinyl Gutter Systems, undated PDF, retrieved 2026-10-10.
About this calculator
Scope and disclaimer
Geometry only: converts a chosen pitch into drop and marking points. Does not size gutters or downspouts or confirm code compliance. Lengths up to 10,000 ft; pitches from level up to 10 in per 10 ft.
This calculator gives information and preliminary estimates only. It is not professional design advice and does not replace the governing code, product instructions or a qualified professional's judgement for your project.