Find a slope's angle, percentage grade, and ratio from rise and run — plus check a road gradient against IRC (Indian Roads Congress) standards, or size a wheelchair ramp.
Key Takeaways
- A gradient can be written three ways for the same slope — as a percentage (5%), a ratio (1 in 20), or an angle (2.86°) — and Indian road signage typically uses the "1 in X" ratio form.
- IRC sets three tiers of road gradient, not one hard limit: ruling (the design target), limiting (used where ruling isn't feasible), and exceptional (short unavoidable stretches only) — each terrain type has all three.
- A wheelchair ramp's length depends entirely on the rise it needs to cover, not a fixed number — at the standard 1:12 slope, every 25mm of rise needs about 300mm of ramp length.
What This Slope Calculator Does
This calculator answers slope and gradient questions in three ways. The general Slope (Rise & Run) mode takes any two measurements and returns the slope as a percentage grade, a "1 in X" ratio, and an angle in degrees — useful for roof pitch, land grading, drainage fall, or a driveway. The Road Gradient (IRC) mode is specific to India: it checks a rise-and-run gradient against the Indian Roads Congress's published ruling, limiting, and exceptional gradient values for plain/rolling, mountainous, and steep terrain, so you can see at a glance whether a proposed road gradient is within normal design practice. The Wheelchair Ramp mode takes the height a ramp needs to overcome and calculates the ramp length required at standard accessibility slopes.
Everything runs entirely in your browser. No measurement you enter is sent to a server, logged, or stored anywhere — close the tab and it's gone. There's no signup, no email wall, and no ads inside the calculator itself. For an actual road, building, or accessibility project, treat this as a quick design check, not a substitute for a qualified engineer's calculations and sign-off.
How to Use This Slope Calculator
1. Slope (Rise & Run)
- Enter the Rise (vertical height).
- Enter the Run (horizontal distance), same unit as Rise.
- Read the percentage grade, ratio, and angle.
2. Road Gradient (IRC)
- Choose the Terrain Type.
- Enter Rise and Run for the road section.
- See whether it's within the Ruling, Limiting, or Exceptional gradient for that terrain.
3. Wheelchair Ramp
- Enter the Rise the ramp needs to overcome.
- Choose a Ramp Slope ratio.
- See the ramp length required.
The Slope Formulas, Explained
- Percentage Grade = (Rise ÷ Run) × 100
- Ratio = 1 in (Run ÷ Rise)
- Angle = arctan(Rise ÷ Run), converted from radians to degrees
- Ramp Length Required = Rise × Slope Denominator (e.g. ×12 for a 1:12 ramp)
Worked example (Basic Slope): Rise = 5m, Run = 100m.
- Percentage Grade = (5 ÷ 100) × 100 = 5%
- Ratio = 1 in (100 ÷ 5) = 1 in 20
- Angle = arctan(0.05) ≈ 2.86°
Worked example (Wheelchair Ramp): Rise = 750mm, Slope = 1:12.
- Ramp Length = 750 × 12 = 9,000mm = 9 metres
IRC Road Gradient Reference Table
Ruling, limiting, and exceptional gradient values by terrain type, as published in Indian Roads Congress highway design guidance.
| Terrain | Ruling Gradient | Limiting Gradient | Exceptional Gradient |
|---|---|---|---|
| Plain / Rolling | 1 in 30 (3.3%) | 1 in 20 (5%) | 1 in 15 (6.7%) |
| Mountainous | 1 in 20 (5%) | 1 in 16.7 (6%) | 1 in 14.3 (7%) |
| Steep | 1 in 16.7 (6%) | 1 in 14.3 (7%) | 1 in 12.5 (8%) |
Note: the maximum gradient permitted at a hairpin bend is 2.5%, regardless of the general terrain-based limits above, because vehicles are already slowing sharply for the curve.
Quick Definitions
Ruling gradient is the design target a road should normally follow for a given terrain, chosen to balance construction cost against ease of travel. Limiting gradient is a steeper value used only in short stretches where the terrain makes the ruling gradient impractical without excessive earthwork, and exceptional gradient is steeper still, reserved for very short, unavoidable sections.
Frequently Asked Questions
They describe the same slope differently. A percentage grade of 5% means the surface rises 5 units for every 100 units travelled horizontally. The equivalent ratio, "1 in 20," means it rises 1 unit for every 20 units travelled — both describe an identical slope, just expressed differently.
Ruling gradient is what a road should aim for under normal conditions. Limiting gradient allows a steeper grade in short, difficult stretches to avoid enormous earthwork costs. Exceptional gradient is a last resort for very short sections where even the limiting value isn't achievable, and is meant to be used sparingly.
Vehicles are already navigating a sharp horizontal curve at a hairpin bend, which demands more of the driver and the vehicle's traction. Capping the gradient at 2.5% there, even in mountainous or steep terrain where the general limit is higher, reduces the combined difficulty of turning and climbing or descending at the same time.
1:12 is the widely used standard for a comfortable, independently usable ramp and is the safest default. Steeper ratios like 1:10 or 1:8 are sometimes permitted for very short rises where space is limited, but they're harder to use unassisted, so 1:12 should be the starting assumption unless your specific code and site conditions justify otherwise.
Yes, the Slope (Rise & Run) mode works for any rise-over-run question, including roof pitch, drainage fall on a flat roof or pipe, or land grading around a foundation — just enter the rise and run in the same unit.
No. Every calculation runs locally in your browser using JavaScript; nothing you enter is transmitted to any server, logged, or saved once you close or refresh the page.