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A cell dilution calculator answers one question: how much of your counted cell suspension do you pipette, and how much medium do you add, to reach a target cells/mL or a target number of cells per well? The arithmetic is a conservation-of-cells equation, C1V1 = C2V2. The part that trips people up is the units: seeding numbers in protocols are usually cells per well or per vessel, while the equation needs cells/mL. This page gives the formula, a sourced per-vessel lookup table of stock volumes, two worked examples and a spreadsheet, R and Python version, so you can calculate without an online form. It is the page to use if you searched for a dilution calculator for cells.
The cell dilution formula (C1V1 = C2V2)
The number of cells you take from the stock equals the number of cells in the final suspension, so:
C1 × V1 = C2 × V2, which rearranges to V1 = (C2 × V2) / C1 and diluent volume = V2 − V1.
- C1: concentration of the counted stock, cells/mL (use the same unit as C2).
- V1: volume of stock to take, mL.
- C2: target concentration, cells/mL.
- V2: final volume of the diluted suspension, mL.
This is arithmetic, not a vendor rule, and it only works when C1 is larger than C2. If C2 comes out larger than C1, the stock must be concentrated (for example by centrifugation and resuspension in less medium) rather than diluted. The same equation applies to molar solutions; for those see mol to molarity, and for percent solutions see the percent solution dilution calculator.
Get a reliable C1 from your count
The result is only as good as the stock concentration. The ATCC Animal Cell Culture Guide describes the hemocytometer as the common way to estimate cell number and viability, and instructs: “Record the number of cells in each section. Average the number of cells, and multiply by the dilution factor.” It also advises adjusting the suspension so that 50 to 100 cells fall in each of the four sections. If you counted after mixing the sample with a dye, include that dilution factor before you use the result as C1. The full counting procedure and the 104 conversion are in the hemocytometer counting and viability guide, and instrument counts are covered in what is an automated cell counter. Use the viable cell concentration as C1 when your protocol specifies viable cells, as the ATCC suspension density range below does.
Convert cells per well to cells/mL
Protocol tables give seeding as cells per vessel or cells per well. To use the formula, divide by the medium volume that the vessel will hold, which is your choice and must be fixed before you calculate:
C2 (cells/mL) = cells per well ÷ medium volume per well (mL)
Example: a 6-well plate seeded at 300,000 cells per well in 2 mL of medium (a volume you choose) gives C2 = 300,000 ÷ 2 = 150,000 cells/mL. Seeding the same 300,000 cells in 3 mL would be 100,000 cells/mL. The cell number is the protocol; the concentration is a consequence of the volume you pick.
Sanity-check ranges
ATCC states that most cultures grow at an initial inoculum of 103 to 104 cells/cm2, and that suspension cultures are seeded at 2 × 104 to 5 × 105 viable cells/mL. ATCC also lists over-dilution as a cause of poor growth: “The cell suspension was diluted below the recommended cell density range.” If your calculated C2 or cells/cm2 is far outside these ranges, recheck the units before you seed. Cell-line-specific densities come from the cell line’s own data sheet.
Lookup table: stock volume per vessel
The seeding numbers are the Thermo Fisher Scientific Gibco “Useful Numbers for Cell Culture” values, which are given for HeLa cells and are cells per vessel (dishes and flasks) or cells per well (plates). They are example values, not universal: the Gibco page notes that the cell count on a confluent vessel varies with cell type. The two right-hand columns are calculated here as cells needed ÷ stock concentration, converted to microlitres (µL). The values divide exactly, so no rounding was needed except the last two plate rows, which are shown to one decimal place. Spot check: 96-well, 10,000 cells ÷ 1,000,000 cells/mL = 0.01 mL = 10 µL.
| Vessel | Surface area (cm2) | Seeding (cells per vessel or well) | Stock volume at 1 × 106 cells/mL (µL) | Stock volume at 5 × 105 cells/mL (µL) |
|---|---|---|---|---|
| 35 mm dish | 8.8 | 300,000 | 300 | 600 |
| 60 mm dish | 21.5 | 800,000 | 800 | 1,600 |
| 100 mm dish | 56.7 | 2,200,000 | 2,200 | 4,400 |
| 150 mm dish | 145 | 5,000,000 | 5,000 | 10,000 |
| 6-well plate | 9.6 | 300,000 | 300 | 600 |
| 12-well plate | 3.5 | 100,000 | 100 | 200 |
| 24-well plate | 1.9 | 50,000 | 50 | 100 |
| 48-well plate | 1.1 | 30,000 | 30 | 60 |
| 96-well plate | 0.32 | 10,000 | 10 | 20 |
| 384-well plate | 0.084 | 1,800 | 1.8 | 3.6 |
| 1536-well plate | 0.03 | 900 | 0.9 | 1.8 |
| T-25 flask | 25 | 700,000 | 700 | 1,400 |
| T-75 flask | 75 | 2,100,000 | 2,100 | 4,200 |
| T-175 flask | 175 | 4,900,000 | 4,900 | 9,800 |
| T-225 flask | 225 | 6,300,000 | 6,300 | 12,600 |
The table is the stock volume only. Add medium to reach the working volume you have chosen for the vessel, and scale up for the number of vessels and any overage (see example 2). Surface areas and seeding densities per cm2 for other cell lines are discussed in cell culture reference numbers.
Worked example 1: dilute a stock to a target concentration
You counted a stock at 2.4 × 106 viable cells/mL and want 10 mL at 2.5 × 105 cells/mL, the re-seeding density in ATCC’s example for adapting cells to suspension culture.
- V1 = (2.5 × 105 × 10 mL) ÷ (2.4 × 106) = 1.0417 mL, rounded to 1.04 mL of stock.
- Diluent = 10 − 1.04 = 8.96 mL of fresh medium.
- Check: 1.04 mL × 2.4 × 106 = 2.50 × 106 cells in 10 mL, which is 2.5 × 105 cells/mL.
Worked example 2: seed a 96-well plate with overage
Target: 60 wells at 10,000 cells per well (the Gibco 96-well value), 100 µL of cell suspension per well (your chosen volume), stock counted at 1.2 × 106 cells/mL, with 10% overage so the last wells are not short. The overage percentage is a practical choice, not a source figure.
- C2 = 10,000 cells ÷ 0.1 mL = 100,000 cells/mL.
- V2 = 60 wells × 0.1 mL × 1.10 = 6.6 mL.
- V1 = (100,000 × 6.6) ÷ 1,200,000 = 0.55 mL of stock.
- Diluent = 6.6 − 0.55 = 6.05 mL of medium.
- Check: 0.55 mL × 1,200,000 = 660,000 cells; 60 × 10,000 × 1.10 = 660,000 cells.
Mix the suspension gently before each aspiration so the cells stay evenly distributed. Pipette handling affects accuracy; see pipetting technique for accuracy and precision. If a calculated stock volume is very small, make an intermediate dilution first, then recalculate from the intermediate concentration. For splitting an existing culture rather than seeding a defined number, see cell passaging: confluency and split ratios.
Compute it in Excel, R or Python
Excel or Google Sheets
Put C1 in A2, C2 in B2 and V2 (mL) in C2. Then stock volume is =B2*C2/A2 and diluent is =C2-B2*C2/A2.
R
dilute <- function(c1, c2, v2) {
stopifnot(c2 <= c1)
v1 <- c2 * v2 / c1
c(stock_mL = v1, diluent_mL = v2 - v1)
}
dilute(1.2e6, 1e5, 6.6)Python
def dilute(c1, c2, v2):
if c2 > c1:
raise ValueError("target is above stock; concentrate instead")
v1 = c2 * v2 / c1
return v1, v2 - v1
print(dilute(1.2e6, 1e5, 6.6)) # (0.55, 6.05)Both functions refuse a target above the stock concentration, which is the usual symptom of a unit mix-up (cells per well entered as cells/mL).
Safety note
Dilution arithmetic does not change the hazards of what you are diluting. Read the safety data sheet (SDS) for any reagent in the suspension, including counting dyes and dissociation or freezing reagents; the SDS concept and employer duties are explained in OSHA SDS requirements. Handle cells at the biosafety level your institution assigns to them and work in a biosafety cabinet where your local rules require it.
FAQ
How many cells per well should I seed?
It depends on the cell line, the plate and the assay. The Gibco table above lists example HeLa values (for instance 10,000 cells in a 96-well and 300,000 in a 6-well). ATCC gives 103 to 104 cells/cm2 as a typical inoculum range for most cultures; take line-specific numbers from your cell line’s data sheet.
Why is my calculated stock volume so small?
Because the stock is much more concentrated than the target. V1 = C2V2/C1 shrinks as C1 grows. Make an intermediate dilution, then calculate from that concentration, or dilute to a larger final volume.
Why does C2 come out larger than C1?
Usually a units error (cells per well used as cells/mL) or a stock that is too dilute. The stock has to be concentrated, not diluted.
Does the dye dilution count?
Yes. ATCC’s counting instructions multiply the average count by the dilution factor, so include any dye or sample dilution before you use the result as C1.
Sources
- ATCC, Animal Cell Culture Guide, checked 2026-10-29.
- Thermo Fisher Scientific, Gibco Useful Numbers for Cell Culture, checked 2026-10-29 (seeding values are for HeLa).
