How is the dilution factor defined?

The dilution factor is the total volume divided by the sample volume, where total volume is sample plus diluent. Mixing 1 part sample with 9 parts diluent gives a total of 10 parts, so the factor is 10, written as a 1:10 or ten-fold dilution.

Is it sample-to-diluent or sample-to-total?

The factor is sample to total. A 1:10 dilution means 1 part sample in 10 parts total, which is 1 part sample plus 9 parts diluent. Confusing the two is the most common dilution error, so this tool always uses sample over total.

How do I get the original concentration back?

Multiply the concentration you measured in the diluted sample by the dilution factor. If a diluted sample reads 5 units per mL at a ten-fold dilution, the original was 50 units per mL. Enter the measured value and the tool does this for you.

Does this work for serial dilutions?

Each step uses this same factor. For a serial dilution, multiply the individual step factors together. Three successive ten-fold steps give an overall factor of 1,000, so multiply your final reading by 1,000 to recover the starting concentration.

Do the volume units matter?

Only that both volumes use the same unit, because the factor is a ratio and the units cancel. Mix microlitres with microlitres or millilitres with millilitres. The concentration unit you enter is simply carried through to the original result.

Dilution Factor & Back-Calculation Tool

Dilutions are everywhere in the lab, and the two questions that follow are always the same: what is the dilution factor, and what was the concentration before I diluted? This tool answers both from the volumes you used.

How it works

The dilution factor is the total volume divided by the volume of sample you started with:

DF       = (sample + diluent) / sample = total / sample
original = measured x DF

The diluted concentration is always lower than the original by exactly this factor, so multiplying a measured diluted reading by the factor recovers the original. For example, 1 mL of sample plus 9 mL of buffer gives a total of 10 mL, a factor of 10. If the diluted sample then reads 5 mg/mL, the original was 5 x 10 = 50 mg/mL.

Tips and notes

The single most common mistake is treating a 1:10 dilution as 1 part sample to 10 parts diluent. It is not: it is 1 part sample in 10 parts total, which means 1 part sample plus 9 parts diluent. For serial dilutions, multiply the per-step factors together, so three ten-fold steps make an overall 1,000-fold dilution. Keep both volume entries in the same unit; the concentration unit is carried straight through to the back-calculated result.