How To Calculate A Gradient For Drainage, Ramps, Slopes Etc

We Show You How To Calculate Slopes & Gradients By Hand and Using Our Calculator

Article By: Tom Ayre
Last Update: May 2020

Calculating a gradient slope is an important element of any building and civil engineering design project. Gradients are crucial to several elements of construction. Typical situations to in which you will need to calculate a gradient are:

  • Pipe drainage fall rates – without a correct gradient the water in the pipe may not flow.
  • Access ramps – standards are very strict on things like disability ramps.
  • Stairs – like access ramps, stairs are a very important element to get right in terms of gradient/slope design.

In this blog article, I will be showing you how to calculate the gradient by hand which will be useful for sites where you don’t have a calculator. We also have a calculator for each section making the process even easier.

Included in this article:

  • What Is A Gradient?
  • How to Calculate a Gradient Number
  • How to Calculate a Gradient Slope
  • How to Calculate a Gradient Ratio
  • How to Calculate Pipe Fall / Rise
  • British Standard Drainage Guidance
  • Free Downloadable Gradient Tools

We have also added a free, downloadable excel spreadsheet to make calculations offline nice and easy! Skip to the end if you want.

Let’s get started!

A Brief Introduction to Gradients

A gradient is a complicated word for quite a simple concept. The gradient refers to the change rate of a slope. Take for instance a gradient of slope that is 1 in 40 (1:40).

A 1:40 slope means that for every 40 metres along the ground, the slope height increases by 1 metre. A 1:0.5 slope means that for every 1 metre along the ground, the slope height increases by 0.5 metres.

A gradient can be expressed in 2 ways, a number or a ratio.

For instance, a 1:40 gradient number is shown as 0.025 (an example is shown in the calculation section).

The same number can be converted into a ratio

For instance, a ratio is shown as “1:40”

The following sketch should help.

gradient of a slope

How to Calculate a Gradient Number

So how do you calculate a gradient? This is very simple.  Just make sure you keep your run/rise units the same (meters, centimetres, miles, km etc)

Step 1: Work out the run length. This is the horizontal distance along the ground. Example number 60 metres.

Step 2: Work out the rise length. This is the vertical length going up. Example number 12 metres.

Step 3: Divide the rise length by the run length, in a calculator this would be 12 ÷ 60. This would equal a gradient of 0.2.

Gradient Number Hand Calculation

Example 1

Run length = 25 metres
Rise Length = 0.8 metres
Gradient = 0.8 / 25
Gradient = 0.032

Example 2

Run Length = 500 mm
Rise Length = 1200 mm
Gradient = 1200 / 500
Gradient = 2.4

Gradient Number Online Calculator

Make sure you use the same units in both the run and the rise calculators. So keep things in metres, millimeters, centimetres etc.

How to Calculate a Gradient Slope %

So how do you calculate a gradient slope %? Again, this is very simple. Just make sure you keep your run/rise units the same (meters, centimetres, miles, km etc)

Step 1: Work out the run length. This is the horizontal distance along the ground. Example number 60 metres.

Step 2: Work out the rise length. This is the vertical length going up. Example number 12 metres.

Step 3: Divide the rise length by the run length, in a calculator this would be 12 ÷ 60. This would equal a gradient slope of 0.2 which is 20% (multiply by 100 if you need to).

Gradient Slope Hand Calculator

Example Calculation 1

Run length = 25 metres
Rise Length = 0.8 metres

Gradient = 0.8/ 25
Gradient %= 0.032

Gradient % = 0.032 x 100
Gradient % =3.2

Example Calculation 2

Run length = 3200 mm
Rise Length = 750 mm

Gradient = 750/ 3200
Gradient % = 0.234%

Gradient % = 0.234 x 100
Gradient % = 23.438%

Gradient Slope Online Calculator

Make sure you use the same units in both the run and the rise calculators. So keep things in metres, millimeters, centimetres etc.

How to Calculate a Gradient Ratio

So how do you calculate a gradient ratio? Another bit of easy maths. Just make sure you keep your run/rise units the same (meters, centimetres, miles, km etc)

Step 1: Work out the run length. This is the horizontal distance along the ground. Example number 60 metres.

Step 2: Work out the rise length. This is the vertical length going up. Example number 12 metres.

Step 3: Divide the run length by the rise length, in a calculator this would be 60 ÷ 12 . This would equal a gradient of 5, this would then be shown as a ratio so = 1:5

Gradient Ratio Hand Calculator

Example Calculation 1

Run length = 25 metres
Rise Length = 0.8 metres
Gradient = 25/ 0.8
Gradient = 1:31.25

Example Calculation 2

Run Length = 500 mm
Rise Length = 1200 mm
Gradient = 500/ 1200
Gradient = 1:0.4166

Gradient Slope Online Calculator

Make sure you use the same units in both the run and the rise calculators. So keep things in metres, millimeters, centimetres etc.

How to Calculate Pipe Fall / Rise

If you know what your pipe needs to fall at (example 1:40, 1:20 etc) then you will need to know what the pipe needs to drop by over the length of the pipe.

Step 1: Work out the run length. This is the horizontal distance along the ground. Example number 60 metres.

Step 2: Work out required fall ratio (usually specified by designer). Example fall is 1:30

Step 3: Divide the horizontal distance by the fall number. Example would be 60 ÷ 30. Your fall in pipe therefore equals 2 metres.

Pipe Fall Rise Hand Calculation

Example Calculation 1

Run length = 25 metres
Fall ratio = 1:80

Pipe fall = 25 ÷ 80
Pipe fall = 0.3125 m

Example Calculation 1

Run length = 3250 mm
Fall ratio = 1:40

Pipe fall = 3250 ÷ 40
Pipe fall = 81.25 mm

Pipe Rise Fall Online Calculator

Make sure you use the same units in both the run and the rise calculators. So keep things in metres, millimeters, centimetres etc.

Outline British Standard Drainage Guidance

Rainwater Drains

Rainwater drains at 75 & 100 mm should be laid at ratios of 1:100 or less.

Rainwater drains at 150 mm should be laid at ratios of 1:150 or less.

Rainwater drains at 225 mm should be laid at ratios of 1:225 or less.

Waste Water Pipes (Self Cleaning)

Flows Less Than 1 litre per second (pipes not exceeding DN100) – should be laid at ratios of 1:40 or less.

Flows More Than 1 litre per second (pipes not exceeding DN100) – should be laid at ratios of 1:80 or less with at least 1 toilet connected to it.

DN150 pipe – should be laid at ratios of 1:150 or less with at least 5 toilets connected to it.

Free Downloadable Gradient Tools

Gradient Calculator Excel Spreadsheet

  • Calculate gradient number
  • Calculate gradient ratio
  • Calculate gradient slope %
  • Microsoft Excel Format
gradient calculator excel spreadsheet

Gradient Calculation PDF Guide

  • Calculating gradient number.
  • Calculating gradient slope.
  • Calculating gradient ratio.
  • Calculating pipe rise / fall.
  • Offline PDF download.

Access Our Download Area

Hopefully, this explains what a gradient is and how it is calculated. If you have any questions or queries I can help with then please let me know in the comments section. Happy to assist!

Related Articles

By |October 12th, 2018|Calculators|2 Comments

2 Comments

  1. Sanam September 13, 2019 at 7:59 pm - Reply

    Gradient can also be expressed in a 3rd way: percentage.
    For example, a 1:40 gradient (i.e., 1 unit rise over 40 unit run) is equal to fraction number 1/40 = 0.025 which in turn is the same as 2.5%.
    Percentage expression for gradient/slope is quite commonly used in civil engineering.

    • Francesca Burke September 20, 2019 at 7:59 am - Reply

      Hi Sanam,
      That’s a good point, thank you! We hope you enjoyed our post. :)

      THS Concepts

Leave A Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.