Tools in this category
Slope calculator
A slope in %, degrees and 1 in X, checked against UK guidance.Pool volume calculator
The water volume in m³ and litres, and the filling time.Stair calculator
Risers, rise, going and the Approved Document K checks.Mitre angle calculator
Mitre cuts, square corners and roof pitch in degrees.Roof area calculator
Roof area and number of tiles for the chosen tile.Wall area calculator
Wall, floor and ceiling areas in m², openings taken off.
Measure first, then apply the rule
Every calculation in this section starts from measurements taken on site. The slope calculator turns a height difference and a distance into a percentage and an angle, and compares them with the minimum for a roof, a drain or a ramp. The stair calculator finds the number of steps that divides the floor-to-floor height into equal rises. The room area calculator takes off doors and windows and passes the figure on to the paint, tile or plasterboard calculators. The roof area is worked out from the plan and the pitch.
Slope: percentage, degrees and ratio
The same slope can be written three ways. As a percentage, it is the centimetres of fall for every metre measured horizontally; in degrees, the angle with the horizontal; as a ratio, written “1 in n” in the UK, the horizontal distance for every unit of fall. The table comes from trigonometry alone (the angle is the arctangent of the gradient), with no figure from any regulation, so it holds in any country.
| Gradient | Angle | Ratio | Fall per metre | Sloping length per horizontal metre |
|---|---|---|---|---|
| 1% | 0.57° | 1 in 100 | 1 cm | 1.000 m |
| 2% | 1.15° | 1 in 50 | 2 cm | 1.000 m |
| 5% | 2.86° | 1 in 20 | 5 cm | 1.001 m |
| 8% | 4.57° | 1 in 12.5 | 8 cm | 1.003 m |
| 10% | 5.71° | 1 in 10 | 10 cm | 1.005 m |
| 30% | 16.70° | 1 in 3.33 | 30 cm | 1.044 m |
| 50% | 26.57° | 1 in 2 | 50 cm | 1.118 m |
| 100% | 45° | 1 in 1 | 1 m | 1.414 m |
A 100% slope is not a vertical wall: it is a 45° angle, because you rise one metre for every metre you move forward.
Which rules apply in the UK
In England, stairs, ramps and guarding are covered by Approved Document K, and drain gradients by Approved Document H, alongside BS EN 12056. Wales has its own Approved Documents, Scotland its Technical Handbooks and Northern Ireland its Technical Booklets, so where a figure differs the page says which nation it applies to. The minimum pitch of a roof depends on the covering and is set by the tile or sheet manufacturer. Each calculator names the document it applies; without an open UK source, the value is not shown.
The most common measuring mistakes
The first is measuring along the slope instead of horizontally: on a roof at 30%, the rafter is 4.4% longer than its plan length, as the last column of the table shows. The second is mixing metres and millimetres in the same formula, which is why the calculators ask for a unit with every measurement. The third concerns stairs: measuring floor to floor on the bare structure and forgetting the floor finish still to be laid, which changes the rise of the first or last step.