Cell count calculator (hemocytometer)
Turn counts per square into cells per mL, with dilution, trypan blue viability and the volume to seed.
One number per square, separated by commas, spaces or new lines. Leave the dead field empty if you did not stain.
How do you calculate cells per mL from a hemocytometer count?
Each large corner square of a standard counting chamber is 1 mm × 1 mm with a coverslip depth of 0.1 mm, so it holds 0.1 mm³ = 10⁻⁴ mL. Dividing the average count per square by that volume is the same as multiplying by 10⁴. Then multiply by the dilution factor to get back to the original suspension: cells/mL = average cells per square × dilution factor × 10⁴. The calculator works out the chamber factor from the square side and depth you enter, so it also covers other square sizes: a small 0.25 mm square at the same depth gives a factor of 160,000. Check your chamber’s markings for the real depth.
Worked example with trypan blue
A cell suspension is mixed 1:1 with trypan blue (dilution factor 2) and the four corner squares are counted. Unstained, live cells: 85, 92, 88, 95 (total 360, average 90); blue, dead cells: 5, 3, 6, 4 (total 18). The viable concentration is 90 × 2 × 10⁴ = 1,800,000 cells/mL, and the viability is 360 ÷ 378 = 95.2%. The totals follow a worked example in the CASRAI hemocytometer guide; the split between squares is ours. About 5% counting error is expected from the 378 cells counted.
How is the dilution factor worked out?
The dilution factor is the total volume divided by the volume of cell suspension: (cell parts + dye or diluent parts) ÷ cell parts. Equal volumes of cells and trypan blue give 2. In the protocol described by AAT Bioquest, 100 µL of cells and 400 µL of 0.4% trypan blue make a total of 500 µL, so the factor is 5. Enter the parts of cell suspension and the parts of dye in the two boxes, and the calculator does the rest. If you also diluted the suspension in medium before adding dye, multiply the two dilution factors yourself.
What counts as live or dead, and which cells do I count at the borders?
Cells with an intact membrane exclude trypan blue and stay clear, so they are counted as viable; cells with damaged membranes take up the dye and look blue, and are counted as dead. Viability is live cells divided by all cells. For cells lying on a square’s border, counting guides use the same rule everywhere: count those touching the top and left lines and skip those touching the bottom and right lines (or the other adjacent pair, as long as you keep to it). Without such a rule the same cell would be counted twice by neighboring squares.
How many cells should I count?
Guides disagree on the target. One vendor note says accurate counts need about 20 to 50 cells in a 1 × 1 mm square; above 50, dilute more, and below 20, use the sample undiluted. The CASRAI guide instead asks for at least about 100 cells in total across the counted squares, because counting error follows Poisson statistics: it is about 1 ÷ √N, so 100 cells give around ±10% and 400 cells around ±5%. The calculator shows the error that goes with your total and warns when you counted fewer than 100 cells. More squares or a repeated loading is the cheapest way to improve it.
How much suspension do I take to seed my plate?
Total cells needed = cells per well × number of wells; the volume of suspension is that number divided by the concentration. To put 100,000 cells in each of 6 wells of 2 mL, from the 1,800,000 cells/mL suspension above, you need 600,000 cells, or 333.3 µL of suspension plus 11.67 mL of medium to make 12 mL. If the suspension is too dilute, the volume needed can exceed the wells’ volume; the calculator then tells you to concentrate the cells by centrifugation. Use the viable concentration if you want live cells in the wells.
Common mistakes and limits
Forgetting the dye dilution is the commonest error and makes a count 2 to 5 times too low. Overfilled or underfilled chambers, air bubbles and clumped cells change the real volume or the count: the factor of 10⁴ assumes the chamber fills to the full 0.1 mm depth under a properly seated coverslip. Cell sizes and counting rules vary between chambers, so follow your chamber’s manual. A manual count is an estimate for routine culture; it is not a validated assay for clinical or manufacturing release. Always count the same sample twice if the numbers decide an experiment.
For research and teaching use; follow your lab’s protocol. Sources (method pages, secondary; the Merck Millipore cell quantification page did not load when we checked): CASRAI, hemocytometer cell counting and viability guide (chamber geometry, formula, border rule, viability, the 360/18 example); AAT Bioquest, trypan blue dye exclusion assay (20–50 cells per square, the 1:5 trypan blue dilution). Related: dilution calculator, serial dilution calculator, doubling time calculator, molarity calculator and standard error calculator.