Last updated: 2026-11-06
Bottom line. Two flasks both marked 100 mL, both called "100 mL volumetric flask", can carry different tolerances — not because one maker cut corners, but because the standard defines two classes. ISO 1042 (GB/T 12806 in China) classes volumetric flasks as Class A and Class B, and Class B tolerance is the wider of the two. Before choosing a class, there are two premises that get overlooked even more often: the calibration basis is 20 °C, and a volumetric flask is calibrated TC (to contain), not to deliver.
[!] This page explains the class system and calibration basis. It does not list numeric tolerances — publicly available sources conflict at the same capacity, and the standard text itself was not obtained. Sources are listed at the foot of the page.
The core difference between classes is one thing: tolerance width.
| Class A | Class B | |
|---|---|---|
| Tolerance | Narrower | Wider |
| Typical use | Standard solutions, regulated testing | Routine prep, teaching, preliminary work |
| Cost | Higher | Lower |
[!] A common misreading: Class B is not "non-conforming". It meets the standard — it simply sits in a wider tolerance band. Class B for teaching or preliminary work is entirely appropriate; Class B for a calibration standard is the wrong choice.
A flask's nominal volume is calibrated at a specified temperature, given in public sources as 20 °C.
Why does temperature change the volume? Because glass itself expands and contracts — borosilicate 3.3 has a coefficient of linear expansion of about 3.3×10⁻⁶ K⁻¹. As temperature moves, the internal cavity changes and the actual volume at the mark shifts.
So "a 100 mL flask" strictly means "a flask that holds 100 mL at 20 °C" (for how temperature relates to limits, see the thermal shock article). Used well away from the calibration temperature, the reading drifts even if the flask itself is perfectly conforming.
[!] The liquid itself usually changes volume more with temperature than the glass does. This page only states that temperature is part of the measurement basis; it does not compute a specific deviation — that needs a specific liquid and temperature range, which we have not calculated.
A volumetric flask is calibrated TC / In (to contain) — the marked volume is what the flask holds.
That differs from TD / Ex (to deliver), where the marked volume is what pours out. The two can give different numbers for the same vessel, because some liquid always remains behind.
So when making up a solution, the correct action is to fill to the mark and mix — not to "pour out 100 mL". If the method document does not state this, the prepared concentration will be systematically low.
Stoppered flasks use ISO 383 joints — common sizes include 10/19, 14/23, 24/29, 29/32 and 34/35. The numbers mean large-end diameter / joint length in mm; see the standard joint article.
So replacement stoppers are matched by joint specification, not by "the neck looks about right".
[!] We have not obtained the ISO 1042 or GB/T 12806 standard text, and publicly available sources give conflicting figures at the same capacity, so this page does not list numbers. For accurate values, consult the standard, or ask a supplier for that product's calibration certificate.
It depends on the uncertainty your work requires. Class B conforms; its tolerance is simply wider. For regulated or traceable results, Class A is normally required.
Because the nominal volume is calibrated at 20 °C. Away from that temperature both the glass and the liquid change volume, so the actual volume at the mark shifts.
[!] We do not give a blanket drying condition — repeated heating may affect volumetric accuracy. Follow the product documentation, or confirm with us.
This page explains the class system and calibration basis and does not list numeric tolerances. Sources below.