What is the C1V1 = C2V2 formula used for?

The C1V1 = C2V2 formula is used to calculate the volume of a concentrated stock solution needed to prepare a diluted solution of known concentration and volume. It applies whenever you need to dilute any solution: cell suspension, reagent, antibody, drug compound, or buffer. The formula works because the number of moles or cells of solute is conserved during dilution.

How do I calculate the dilution factor for a serial dilution?

The dilution factor for a single serial dilution step is the ratio of total volume to stock volume: DF = V(total) / V(stock). For a step where 0.1 mL of stock is added to 0.9 mL of diluent (total 1.0 mL), DF = 1.0/0.1 = 10. The cumulative dilution factor after n steps is DF raised to the power n. For a 6-step 1:10 serial dilution, the final tube is diluted 10^6-fold from the original stock.

What volume should I use per step in a serial dilution?

The optimal step volume depends on the application. For plate counts with bacteria, 1 mL per step is standard, allowing 0.1 mL plating volume. For 96-well plate assays, 100-200 uL per well is typical. For ELISA standard curves, 200-250 uL per step allows duplicate or triplicate measurements. Smaller volumes can be used to conserve precious samples but introduce greater pipetting error.

How do I convert between cells/mL, OD600, and CFU/mL?

There is no universal conversion between OD600 and cell count because the relationship is strain-specific, growth-phase-dependent, and spectrophotometer-dependent. As a general guide, for E. coli in LB at mid-exponential phase, OD600 = 1.0 corresponds to approximately 8 x 10^8 cells/mL. However, every lab should generate its own calibration curve by measuring OD alongside plate counts.

What is the difference between a dilution and a solution preparation?

A dilution starts with an existing concentrated solution and reduces its concentration by adding diluent, using C1V1 = C2V2. A solution preparation starts with a dry powder or neat liquid and calculates the mass or volume needed to achieve a target concentration in a given volume: mass (g) = concentration (mol/L) x volume (L) x molecular weight (g/mol). For cell biology, a dilution is appropriate when preparing working concentrations from stock solutions.

Cell Dilution Calculator - C1V1=C2V2 and Serial Dilution Tool

Cell Dilution Calculator

The Cell Dilution Calculator provides two modes: single dilution using the C1V1 = C2V2 formula to find the exact volume of stock and diluent needed, and serial dilution mode that generates a complete step-by-step table showing concentrations, stock volumes, and diluent volumes for each dilution step.

What Is Cell Dilution?

Cell dilution refers to the process of reducing the concentration of cells, microorganisms, or any solute by adding a known volume of diluent (typically a physiological buffer, culture medium, or water). Accurate dilution is essential in cell culture for seeding assays, in microbiology for plate counts, in biochemistry for preparing standards, and in pharmacology for drug dose-response experiments.

The C1V1 = C2V2 Dilution Formula

The fundamental dilution equation states that the amount of solute before dilution equals the amount after dilution:

C1 x V1 = C2 x V2

Where C1 is the initial concentration, V1 is the volume of stock to take, C2 is the desired final concentration, and V2 is the total final volume after dilution.

To find the volume of stock needed: V1 = (C2 x V2) / C1

The volume of diluent to add is: V(diluent) = V2 - V1

Serial Dilutions Explained

Serial dilution is the process of diluting a sample in multiple equal steps. Each step uses a fixed dilution factor, and the output of each step becomes the input for the next. A 10-fold serial dilution (1:10) from a 10^6 cells/mL stock gives concentrations of 10^5, 10^4, 10^3, 10^2, 10^1, and 10^0 cells/mL in successive steps.

Serial dilutions are used to: prepare standard curves for ELISA, Bradford assay, or BCA protein assay; determine minimum inhibitory concentration (MIC) of antibiotics; enumerate bacteria by plate count (CFU method); and prepare dose-response curves for drug assays.

Dilution Factor vs. Dilution Ratio

The dilution factor is the ratio of final volume to stock volume taken (V2/V1). A 1:10 dilution has a dilution factor of 10. The cumulative dilution factor after n serial steps each with factor f is f raised to the power n. After 6 steps of 1:10 dilution, the cumulative factor is 10^6, and a stock of 10^6 cells/mL becomes approximately 1 cell/mL.

Practical Tips for Accurate Dilutions

Always use calibrated volumetric pipettes. Systematic pipetting error compounds in serial dilutions.
Mix thoroughly between each step by pipetting up and down 10 times or by vortexing briefly before transferring.
Change tips between each serial dilution step to avoid carryover.
Work quickly to minimise evaporation, especially with small volumes under 100 uL.
For plating assays, plate within 30 minutes of the final dilution to prevent cell sedimentation or death.

Case Study: Preparing a Standard Curve for ELISA

A researcher is preparing an 8-point standard curve for an IL-6 ELISA starting from a 2000 pg/mL stock. Each point uses a 2-fold serial dilution (1:2) with a final volume of 200 uL per well. Using the cell dilution calculator with serial dilution mode, dilution factor 2, 8 steps, and 200 uL per step: Step 1 gives 1000 pg/mL, Step 2 gives 500 pg/mL, and the full curve covers from 1000 pg/mL down to 7.8 pg/mL. This approach connects directly with our Protein Concentration Calculator, which can verify the stock concentration before the standard curve is prepared.

Counting Cells After Dilution

For plate count methods, the recommended approach is to plate dilutions expected to yield 30-300 colonies per plate. Plates with fewer than 30 colonies are statistically unreliable; plates with more than 300 colonies are too crowded to count accurately. Using the dilution table, select the step that will give a countable plate based on the expected cell density. The CFU/mL in the original sample is then: CFU/mL = colonies counted x dilution factor.

Cell Dilution Calculator