Dilute Back Down Calculator

Keep your existing solution and calculate the diluent needed to lower its concentration. You can also specify newly added diluent as a percentage of the starting or final volume.

Dilute an existing solution

Either lower a known concentration or add a specified amount of diluent without entering a concentration.

Try an example
Calculation type
C₁ × V₁ = C₂ × V₂

Concentration basis: Use molarity (M, mM, µM, nM, pM) or % v/v throughout a calculation. % v/v is the volume of a liquid component per 100 volume units of final solution; it is not % w/v or % w/w. These tools do not convert between concentration categories or percentage bases. Solute must be conserved. Calculated added volumes and mixture concentrations assume additive solution volumes; measure the final volume for precise preparation.

Lower concentration without discarding solution

If a prepared solution is too concentrated, keep its solute and increase its volume with a solute-free diluent. From C₁V₁ = C₂V₂, the required final volume is V₂ = V₁C₁/C₂. Subtract the volume you already have:

Vadded = V₁(C₁/C₂ − 1)

  • C₁: starting concentration.
  • C₂: lower target concentration, greater than zero.
  • V₁: starting solution volume.
  • Vadded: diluent volume to add, assuming additive volumes.
  • V₂: resulting total volume, V₁ + Vadded.

Worked buffer-dilution example

You have 500 µL of a 100 mM buffer and need 25 mM. The dilution factor is 100/25 = 4. The total volume must be 500 × 4 = 2,000 µL. Add 500 × (4 − 1) = 1,500 µL of diluent, giving 2,000 µL total.

Do not add the calculated final volume as diluent: adding 2,000 µL to the original 500 µL would give 2,500 µL at 20 mM. Choose a compatible diluent; dilution changes buffer capacity and can also change pH through activity effects or interactions with the diluent.

Added diluent as a percentage: choose the denominator

This separate mode describes the newly added diluent volume, not the solute concentration or the diluent already present. Let f be the entered percentage divided by 100.

  • Percentage of starting volume: Vadded = V₁f. Adding 10% to 1,000 µL means adding 100 µL, for 1,100 µL total.
  • Percentage of final volume: Vadded = V₁f/(1 − f). For newly added water to be 10% of the final mixture starting from 1,750 µL, add about 194.44 µL, for 1,944.44 µL total.

The final-volume percentage must be below 100%; a starting-volume percentage may be 100% or more. Both estimates assume additive volumes. For concentration-based dilution, match molarity units or use % v/v consistently for the same component. A percentage of added diluent is not a substitute for the solution’s % v/v concentration.