Beaker, Flask or Graduated Cylinder: Which One Measures Accurately?

A beaker used to estimate 1,000 mL may be off by roughly 50 mL, so the actual volume could be closer to 950 mL or 1,050 mL. Beakers and the everyday flasks on a lab bench are made for holding, mixing and heating liquids, and their printed volume marks are a rough guide, not a measurement. When the volume has to be right, you reach for different glassware: a graduated cylinder, a volumetric flask or a pipet. Here is how to tell which one you need.

The Short Answer

The rule behind the list: if the step needs a precise volume you can trust, you want a measuring instrument. If it does not, you want a container.

Why a Beaker Is Not a Measuring Instrument

The graduation marks on an ordinary beaker are accurate to roughly plus or minus 5 percent of full capacity, which is why those graduations are described as approximate. On a 1,000 mL beaker, 5 percent is 50 mL either way. For rinsing, dissolving, heating or holding a solution, that does not matter. For measuring out 1,000 mL that you need to hit, it matters a great deal.

This is not a defect. A beaker is shaped for its purpose: a wide mouth for stirring and adding solids, straight sides that are easy to clean, and a pour spout for decanting. None of that is good for reading a liquid level precisely, and a beaker is not intended for precise volume measurement. Treat the scale on the side as a convenience and nothing more. Avogadro's Lab Supply carries Griffin beakers from 5 mL to 5,000 mL and Berzelius beakers from 100 mL to 600 mL.

Erlenmeyer, Round Bottom and Flat Bottom Flasks: Containers, Not Measures

An Erlenmeyer flask is a conical flask with a flat base and a narrow neck. The taper lets you swirl a solution hard without it climbing the walls, which is why Erlenmeyer flasks are the usual vessel for titrations, for mixing and for any step where splashing would result in lost sample. The narrow neck also takes a stopper.

Round bottom flasks and flat bottom flasks are heating vessels. The round base suits reflux and distillation work; the flat base version does the same job where the flask can stand on its own.

All of these flasks carry capacity marks, and like a beaker's, those marks are approximate. They tell you roughly how much the flask holds, not how much is in it. The 1,000 mL Erlenmeyer flask listing, for example, gives an approximate error of plus or minus 5 percent of full capacity. Do not read a volume off an Erlenmeyer or a boiling flask when the number matters.

Graduated Cylinders: The Everyday Measuring Tool

A graduated cylinder is the first piece of glassware to reach for when a volume has to be accurate. It is tall and narrow by design, so a small change in volume moves the liquid level a long way up the scale, and the scale is divided finely enough to read it. That geometry is the whole difference between a cylinder and a beaker.

Graduated cylinders are measuring instruments, and their listings state a tolerance. A Class A 1,000 mL graduated cylinder, for example, is listed at plus or minus 3.0 mL, against the roughly 50 mL you could be off by in a 1,000 mL beaker. Use a cylinder to measure a reagent, a solvent or a portion of a solution for the next step, then do the mixing and heating in a beaker or flask.

Volumetric Flasks and Pipets: When the Volume Has to Be Right

A volumetric flask is not an ordinary flask. It has a bulb, a long narrow neck and a single line, and it is calibrated TC to contain its marked volume, within the stated tolerance, when filled to that line. Volumetric flasks exist for one job: the quantitative preparation of a solution of known concentration. You add the solute, dissolve it, then fill to the line. There are no other graduations because the flask is not meant to measure anything other than its one marked volume.

A volumetric pipet is the matching tool for moving a fixed measured volume into another vessel. It is calibrated to deliver its marked volume, within its listed tolerance, when used correctly. For measuring variable volumes rather than one fixed volume, a Mohr measuring pipet carries a graduated scale along its length.

Keep the distinction straight when ordering. An Erlenmeyer flask and a volumetric flask are both called flasks, but only one of them is a measuring instrument, and ordering one when you need the other is an easy mistake to make.

What Class A and Class B Mean

Volumetric glassware is graded, and the grade describes its accuracy and tolerance. The exact tolerance for each graded item is specified in its product listing, so you can compare before you buy.

Two listings from the catalog show how fine the differences get. A 1 mL Class A volumetric flask carries a tolerance of plus or minus 0.01 mL. A 1 mL Class A volumetric pipet carries a tolerance of plus or minus 0.006 mL, because it is designed to deliver its volume accurately. Both are far tighter than anything a beaker can do. If your work is reported to a tolerance, buy to that tolerance. If it is not, Class B or student grade glassware may be appropriate when the listed tolerance meets the needs of your procedure.

Every vessel on this page is stocked at Avogadro's Lab Supply, from beakers and Erlenmeyer flasks to graduated cylinders, volumetric flasks and pipets. If you are unsure which piece suits your procedure, contact us with the volume and the step you are doing and we will point you to the right item. More reference material is in our Lab Reference Guides.