What Is a Hemocytometer? Uses & How It Works

Last verified: October 6, 2026. A hemocytometer is a thick, specially marked glass slide used to count cells under a microscope. It holds a precisely known volume of liquid over an etched grid, so a researcher can count the cells that fall within the grid and calculate how many cells are in each milliliter of the original sample. Despite the name, which comes from its original use in counting blood cells, it is now a routine tool in cell culture, microbiology, and many other labs that need to know how many cells they are working with.

The problem a hemocytometer solves is simple but important: cells in a suspension are invisible to the eye and cannot be measured by volume or weight in any useful way. Yet experiments depend on knowing cell numbers. Seeding a culture dish at the right density, setting up an assay with an equal number of cells per well, or deciding how much to dilute a stock all require a count. A hemocytometer gives that count with nothing more than a microscope, a pipette, and a small drop of sample.

What a Hemocytometer Looks Like

A standard hemocytometer is a heavy rectangular glass slide with one or two polished counting chambers in the middle. Each chamber has an engraved grid of fine lines, and the chamber is bounded by raised ridges so that a flat glass coverslip rests at a fixed, known height above the grid. That fixed gap is the key design feature. Because the area of each grid square and the depth of the chamber are both known, the volume of liquid sitting over any square is also known.

• The grid. The engraved pattern is divided into large squares and smaller subdivisions, with some lines doubled or tripled to help the user see which cells count as inside a square. Counting conventions tell you which cells touching a boundary line to include and which to leave out, so that no cell is counted twice.
• The coverslip. A special thick coverslip is used, not an ordinary thin microscope coverslip. Properly seating it so it rests evenly on the ridges is part of what makes the volume calculation valid.
• The loading edge. The sample is introduced at the edge of the coverslip, where capillary action draws it into the chamber without trapped air bubbles.

How the Counting Works, in General Terms

The workflow follows a few steps, and the logic is worth understanding even if you never count cells yourself:

• Prepare the sample. The cell suspension is mixed so the cells are evenly distributed, and is often diluted so the cells are sparse enough to count accurately. The dilution factor must be recorded because it goes into the final calculation.
• Load the chamber. A small volume is drawn into the chamber under the coverslip.
• Count under the microscope. The user counts the cells within the designated grid squares, applying the boundary rules consistently.
• Calculate. The average count per square is multiplied by a conversion factor based on the known chamber volume, and then by the dilution factor, to give cells per milliliter of the original suspension.

Counting several squares and averaging them reduces the effect of random variation, since cells are never perfectly evenly spread. Many labs also repeat the count on a second chamber or a second loading to check agreement.

Live and Dead Cells: Viability Counts

A hemocytometer is frequently used together with a dye that distinguishes live cells from dead ones. In the classic approach, a dye that is excluded by cells with intact membranes is mixed into the sample before loading. Dead or damaged cells take up the dye and look different from healthy cells, so the user can count the two groups separately and report the percentage of viable cells alongside the total. Viability matters because a culture can contain plenty of cells and still be in poor condition, and many downstream experiments require a minimum level of viability. For a step-by-step treatment of the technique itself, see the CASRAI guide on hemocytometer cell counting and viability.

Who Uses a Hemocytometer

Hemocytometers are found in a wide range of settings:

• Cell culture labs use them to count cells before passaging, seeding plates, or freezing stocks. See the beginner overview in cell culture basics for where counting fits in the routine.
• Microbiology and yeast work use counting chambers to estimate the density of microbial cultures, especially for larger cells such as yeast.
• Hematology and clinical laboratories historically used them for blood cell counts, and they are still used in some settings for specific samples and for teaching, though automated analyzers handle most routine clinical counting.
• Teaching labs use them because the method is inexpensive, visual, and builds an understanding of what a cell concentration actually represents.

How a Hemocytometer Differs From Adjacent Equipment

Several other instruments also count or characterize cells, and it helps to know where each fits:

• Automated cell counters use imaging or electrical sensing to count cells faster and with less operator-to-operator variation. They are convenient for high sample volumes, but they cost more and still benefit from an occasional manual check. A hemocytometer remains the reference method many labs use to confirm an automated counter is behaving sensibly.
• Flow cytometers measure many properties of individual cells as they stream past a detector, including size, internal complexity, and fluorescent markers. They can also report counts, but they are a far larger and more expensive instrument built for analysis rather than quick counting. See what a flow cytometer is.
• Spectrophotometers estimate cell density in some cultures by measuring how much light a suspension scatters, which is fast but indirect and gives a relative value rather than a direct cell count.
• An ordinary microscope slide holds a sample but has no fixed volume or grid, so it cannot be used to calculate concentration.

Practical Notes for Research-Administration and Lab-Management Readers

• Procurement. A hemocytometer is a low-cost, durable item, and most labs keep several. The more consequential purchasing decision is whether a lab also needs an automated counter, which depends on how many samples are counted per week and how much consistency matters between users.
• Consumables and care. The chamber and coverslip must be cleaned carefully after each use, since residue or scratches can distort the volume or obscure the grid. Chambers that are chipped, scratched, or hard to clean are usually replaced rather than repaired.
• Training and consistency. Counting is a skill. Results can differ between people who apply the boundary rules differently or who load the chamber unevenly. Labs that depend on accurate counts often write a short standard procedure so everyone counts the same way.
• Biosafety. Cell suspensions, particularly those from human or animal sources or containing infectious agents, call for appropriate handling, disposal, and decontamination of slides and coverslips according to the institution’s biosafety rules.

The Short Version

A hemocytometer is a gridded glass slide that holds a known volume of liquid, letting a researcher count cells under a microscope and calculate how many cells are in each milliliter of a sample. It is inexpensive, direct, and still widely used, both on its own and as a check on automated counters. Its accuracy depends on good mixing, correct loading, and consistent counting rules. For more on selecting and managing instruments like this one, see CASRAI’s broader laboratory equipment and instrumentation coverage.

This page is general information only. It is not clinical or safety training. Follow your institution’s procedures and the manufacturer’s instructions.

Frequently Asked Questions

What is a hemocytometer used for?

It is used to count cells in a liquid sample under a microscope and to calculate the concentration of cells per milliliter. Labs use the result to seed cultures at a consistent density, prepare assays, and track growth.

Why is it called a hemocytometer?

The name comes from its original use in counting blood cells. The same device is now used for many other cell types, such as cultured mammalian cells and yeast.

How does a hemocytometer know the volume it is counting?

The coverslip rests on raised ridges at a fixed height above an engraved grid of known dimensions. Because the area of each square and the depth of the chamber are known, the volume over each square is known too.

Is a hemocytometer more accurate than an automated cell counter?

Not necessarily. A hemocytometer is a direct, well-understood method, but it is slower and depends on operator technique. Automated counters are faster and often more consistent between users. Many labs use the hemocytometer to verify the automated counter.

Can a hemocytometer tell live cells from dead cells?

Yes, when it is used with a dye that live cells exclude and dead cells take up. The user counts the stained and unstained cells separately to estimate viability.

Why does the sample need to be mixed and sometimes diluted?

Cells settle and clump, so an unmixed sample gives an unrepresentative count. Dilution spreads the cells out enough to count them reliably, and the dilution factor is then built into the final calculation.