Free swell index from the soil volumes in distilled water and in kerosene (IS 2720 Part 40).
Water ContentSpecific GravitySieve Analysis (Soil)Liquid & Plastic LimitPermeability (Falling Head)Permeability (Constant Head)Vane Shear
Free Swell Index of Soil Calculator: Free Online Tool
The free swell index calculator gives a simple measure of how much a soil swells when it is placed in water. You enter the volume of soil settled in distilled water, Vd, and the volume of soil settled in kerosene, Vk, both in millilitres. The tool returns the free swell index as a percentage.
Geotechnical technicians, students and site engineers use this index as a quick screening test for expansive soils, such as black cotton soil and some clays. Such soils expand when wet and shrink when dry, causing cracks in floors, boundary walls and light structures. The test is quick, uses simple glassware and helps to decide whether further investigation or special foundation measures may be needed. The calculator follows the procedure of IS 2720 Part 40.
When to use this calculator
Use it after reading the settled volumes in the two measuring cylinders after 24 hours. It is helpful during a soil investigation for a building, a road subgrade or a school boundary wall, when you need to compare several samples from different locations and want a fast number for each of them before choosing which samples go for detailed testing. It also gives a quick check on quotations and site requests before work starts. It also helps students understand how a clay's reaction to water can be expressed with a single number from a simple laboratory test.
How to use the Free Swell Index of Soil Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Enter your values in the input boxes: Volume in distilled water, Vd (ml), Volume in kerosene, Vk (ml). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
The free swell index is calculated as (Vd - Vk) / Vk x 100. In the test, 10 g of oven-dried soil passing the 425 micron sieve is poured into each of two cylinders, one filled with distilled water and the other with kerosene. Kerosene does not cause the soil to swell, so Vk is the base volume. The settled volume is read after 24 hours. For the default values of 15.0 ml in water and 10.0 ml in kerosene, the index is (15 - 10) / 10 x 100 = 50.0 percent.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| Free swell | |
|---|---|
| Volume in distilled water (Vd) | 15.0 ml |
| Volume in kerosene (Vk) | 10.0 ml |
| Free swell index | 50.0 % |
Tips and common mistakes
- Use oven-dried soil passing the 425 micron sieve and weigh 10 g for each cylinder, so that the two readings are comparable.
- Read the settled volumes after the full 24 hours and avoid disturbing the cylinders while the soil is settling.
- Use distilled water in one cylinder and kerosene in the other, and make sure both cylinders are the same size and clean.
- Enter Vd and Vk in millilitres, and check that both are positive, since the calculator rejects zero or negative values.
- Do not decide a foundation solution from this index alone; use it as a screening value and confirm with other soil tests and the engineer.
Frequently asked questions
What is the formula for free swell index?
Free swell index equals (Vd - Vk) divided by Vk, multiplied by 100. Vd is the soil volume in distilled water and Vk is the soil volume in kerosene, both in millilitres.
What soil quantity is used in the test?
The calculator note says that 10 g of oven-dried soil passing the 425 micron sieve is poured into each cylinder. The settled volume is read after 24 hours.
Why is kerosene used?
Kerosene is a non-polar liquid that does not cause the soil to swell, so the volume in kerosene shows the base volume of the soil without swelling. Water causes the expansion that is measured.
What does an index of 50 percent mean?
It means the soil volume in water is 50 percent greater than in kerosene, as in the default example of 15 ml and 10 ml. The calculator gives the number only, so interpret it with your project criteria.
Can the index be negative?
Mathematically it can if Vd is less than Vk, but this is unusual and normally indicates a reading error or poor settlement. Repeat the test and check the cylinders if you see it.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
