Road Grade Calculator
Enter rise and run, station elevations, or a known grade to compute grade percentage, slope angle, rise-to-run ratio, AASHTO highway compliance, ADA ramp limits, cycling difficulty, and truck grade resistance.
Calculating grade…
Enter your measurements above and click Calculate Grade.
What Is Road Grade Percentage?
Road grade — also called slope percentage, gradient, or incline — expresses how much a road rises or falls vertically for every 100 units of horizontal distance. A 5% grade means the road climbs 5 metres for every 100 metres of horizontal run. It is the most widely used measure in highway engineering because it directly relates to vehicle performance, fuel consumption, and safety.
The formula is straightforward: Grade (%) = (Rise ÷ Run) × 100. Rise is the vertical change in elevation and Run is the horizontal distance. When rise is positive, the road is ascending; when negative, it is descending. The absolute value of grade is used when comparing against design standards.
Closely related measures include the slope angle in degrees — calculated as arctan(grade/100) — and the rise-to-run ratio (written as 1:x). Engineers in road design, drainage planning, and accessibility compliance all use grade percentage daily.
Road Grade Diagram
Visual explanation of rise, run, grade percentage, and slope angle on a road cross-section.
Right-Triangle Model
Grade Intensity Scale
AASHTO Maximum Grade Standards
AASHTO's A Policy on Geometric Design of Highways and Streets (Green Book) sets maximum grade limits based on design speed and terrain type. These values protect stopping sight distance, prevent truck crawl, and maintain driver comfort. The table below summarises the most common design speed thresholds.
| Design Speed (km/h) | Level Terrain Max Grade | Rolling Terrain Max Grade | Mountainous Terrain Max Grade |
|---|---|---|---|
| 50 | 7% | 9% | 12% |
| 60 | 6% | 8% | 11% |
| 70 | 6% | 7% | 10% |
| 80 | 5% | 6% | 9% |
| 100 | 4% | 5% | 7% |
| 120 | 3% | 4% | 5% |
Source: AASHTO Green Book (7th edition). Values apply to freeways and expressways. Local roads and ramps may use higher limits.
Grade Percentage, Angle & Ratio Equivalence Table
Different disciplines use different notations. Civil engineers prefer grade percentage; surveying sometimes uses rise-to-run ratio; ramp accessibility is measured in angle degrees. Use the table below to convert between notations quickly.
| Grade % | Angle (°) | Ratio (1:x) | Rise per km (m) |
|---|---|---|---|
| 1% | 0.57° | 1:100 | 10 m |
| 2% | 1.15° | 1:50 | 20 m |
| 5% | 2.86° | 1:20 | 50 m |
| 8.33% | 4.76° | 1:12 | 83 m (ADA max) |
| 10% | 5.71° | 1:10 | 100 m |
| 15% | 8.53° | 1:6.67 | 150 m |
| 20% | 11.31° | 1:5 | 200 m |
| 45% | 24.23° | 1:2.22 | 450 m |
Truck Grade Resistance by Slope
Grade resistance is the additional tractive force a vehicle must generate to maintain speed on an incline. For trucks, the rule of thumb is 20 lb per ton of gross vehicle weight per 1% grade (approximately 98 N/tonne per 1% grade in SI). Steep grades require longer climbing lanes to prevent trucks from slowing traffic below minimum operating speed.
| Grade % | Resistance (lb/ton) | Resistance (N/kN) | Climbing Lane Needed? |
|---|---|---|---|
| 2% | 40 | 0.39 | No |
| 4% | 80 | 0.78 | Study required |
| 5% | 100 | 0.98 | Likely |
| 6% | 120 | 1.18 | Yes (over 500m) |
| 8% | 160 | 1.57 | Yes (over 300m) |
| 10% | 200 | 1.96 | Yes (any length) |
ADA Ramp Slope Requirements
The Americans with Disabilities Act (ADA) and the UK Equality Act set maximum slope gradients for accessible ramps and pathways. A walking surface steeper than the prescribed limits requires a handrail and landing and may disqualify the facility from ADA compliance.
| Surface Type | Max Grade (ADA) | Max Grade (UK) | Notes |
|---|---|---|---|
| Accessible route (walking) | 5% | 5% | No handrail needed |
| Ramped walkway | 8.33% | 8.33% | Handrails required |
| Cross-slope | 2% | 2% | Drainage & wheelchair stability |
| Curb ramp (perpendicular) | 8.33% | 8.33% | Flare ≤ 10% |
Frequently Asked Questions
What does 10% road grade mean?
How do I convert slope percentage to degrees?
What is the maximum grade for an ADA wheelchair ramp?
How is grade resistance calculated for trucks?
What is the difference between grade and gradient?
How do I calculate grade from station elevations?
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