Record delivered volumes to two decimal places (for example, 5.00 mL or 10.00 mL). Both pipets are Class A volumetric ware with a stated tolerance of ±0.02 mL at 20 °C, so the hundredths digit is the last place that carries real meaning. Reporting 5.0 mL discards a legitimate digit; reporting 5.000 mL claims precision the glass cannot support.
Tolerance defines the last significant decimal
The decimal question is really a tolerance question wearing a costume. A volumetric pipet is calibrated to deliver one nominal volume within a published error band, and that band sets how far right of the decimal point your recorded number can honestly go.
For Class A pipets in the 5 mL and 10 mL sizes, the conventional tolerance is ±0.02 mL at 20 °C, the reference temperature for glassware calibration. The number sits in the hundredths column, so the hundredths column is the smallest digit you are entitled to report. The mechanism is geometric: the pipet's accuracy depends on the bore and the etched ring mark, and manufacturing can hold those features only so precisely. The tolerance is the maker's quantified admission of that limit.
There is a second mechanism, and it is the one most people miss. A pipet marked "to deliver" (TD) is calibrated so that the residual film of liquid left clinging to the inner wall after draining is already accounted for in the stated volume. The film is not an error to correct; it is baked into the calibration. This is why you never blow out a TD pipet or rinse the last drops into the receiving vessel. Doing so adds liquid the calibration assumed would stay behind, and your recorded volume becomes quietly wrong by roughly the film volume.
A third mechanism concerns meniscus reading. You set the fill by aligning the bottom of the meniscus with the etched line at eye level, and the precision of that alignment is on the order of a few hundredths of a milliliter. The reading process itself cannot support a thousandths digit.
Plain-language version: the glass was built to hit the target within about two hundredths of a milliliter, so two decimal places is the deepest honest record of what it delivered.
The arithmetic that fixes two decimals
Significant-figure rules point to the same answer. The tolerance digit is the uncertain digit, and by convention you record one uncertain digit and no more. For a ±0.02 mL tolerance, the uncertain digit is the hundredths place.
Absolute versus relative error tells the same story twice. At 5.00 mL, a ±0.02 mL tolerance is a relative uncertainty of about 0.4 percent. At 10.00 mL, the same absolute tolerance is about 0.2 percent. Both land in the range where two decimal places remain meaningful; a third decimal place would imply an uncertainty near 0.02 percent, which no Class A pipet of this size can back up.
The round-number coincidence is worth noticing because it keeps the rule simple across both pipets: 5.00 mL and 10.00 mL are recorded identically in decimal structure, even though their relative uncertainties differ by a factor of two.
Edge cases that shift or preserve the rule
Temperature is the most common edge case. Pipets are calibrated at 20 °C, and both the glass and the liquid expand or contract away from that temperature, so a pipet used at 25 °C delivers a slightly different true volume than its stamp promises. The two-decimal recording convention still holds; what changes is the accuracy of the nominal value, not the number of digits you are entitled to write. If your procedure requires temperature correction, the corrected value is still reported to the hundredths place, because the correction is small relative to the tolerance.
Viscous liquids are a different problem. A solution thicker than water drains more slowly and leaves a heavier film, so the delivered volume can drift outside the aqueous calibration. Again the decimal convention does not change; the pipet's suitability for that liquid does. Slow drainage and incomplete delivery are accuracy failures, not formatting failures.
Partial delivery is the case that tempts people to add digits. If you deliver only part of the pipet's contents, the delivered volume is no longer the calibrated nominal value, and you are estimating rather than measuring. In that situation the honest record is still bounded by the pipet's tolerance, not by how carefully you eyeballed the partial drain.
Air bubbles and a chipped tip preserve the decimal rule but destroy its meaning. A bubble displaces liquid and a damaged tip alters the drain geometry; either can push the delivered volume past ±0.02 mL while your record still reads 5.00 mL. The format is correct and the measurement is not.
The professional caveat worth stating plainly: two decimal places is the correct format for a properly used, properly functioning pipet. It is not a certificate that the delivery was accurate. Format and accuracy are separate claims, and only one of them is guaranteed by the way you write the number.
Class A versus Class B and other glassware
The comparison table below reconciles the alternatives people actually encounter in a lab.
| Vessel or device | Typical stated tolerance | Sensible recorded decimals | Notes |
|---|---|---|---|
| 5 mL Class A volumetric pipet | ±0.02 mL | Two (5.00 mL) | TD calibration; do not blow out |
| 10 mL Class A volumetric pipet | ±0.02 mL | Two (10.00 mL) | Same hundredths digit as the 5 mL |
| 5 mL Class B volumetric pipet | Roughly ±0.04 mL | One to two | Tolerance typically double Class A |
| 10 mL graduated cylinder | About ±0.1 mL | One (10.0 mL) | Reading between marks limits precision |
| 50 mL buret | About ±0.05 mL per reading | Two | Meniscus estimation dominates |
| Micropipette, 100–1000 µL | Device-specific, often ±1% or tighter | Device-specific | Follow the calibration certificate |
The table's lesson is that decimal places should track the tolerance column, not the nominal volume column. A 10 mL graduated cylinder and a 10 mL pipet share a number but not a precision, so they should not share a decimal format.
Time course: from fill to recorded value
The timeline table shows where precision is won or lost across a single delivery.
| Stage | Action | Precision impact |
|---|---|---|
| Rinse | Rinse with the solution, not water | Prevents dilution error before the mark |
| Fill | Draw above the mark, then adjust | Coarse control; overshoot is recoverable |
| Set meniscus | Align bottom of meniscus to line at eye level | Few hundredths of a mL of uncertainty |
| Drain | Let it drain freely into the vessel | Film retention is part of TD calibration |
| Touch off | Touch tip to vessel wall | Completes delivery; do not blow out |
| Record | Write to two decimal places | Matches the ±0.02 mL tolerance |
The stage that most often breaks the chain is the free drain. Tilting the pipet, forcing the last drop, or rushing the drain all change the delivered volume after the meniscus was set perfectly.
Misconceptions worth correcting
The first misconception is that more decimals means more precision. Writing 5.000 mL does not make the delivery more accurate; it makes the record less honest. The trailing zeros assert a thousandths-place confidence that the ±0.02 mL tolerance contradicts.
The second misconception is that the 5 mL pipet deserves fewer decimals than the 10 mL pipet because it is smaller. The tolerances are equal, so the decimal count is equal. Relative uncertainty differs, but absolute formatting does not.
The third misconception is that you should blow out the pipet to get "all" the liquid. For TD pipets, the residual film is calibrated in; blowing it out over-delivers. The list below flags the errors that most often pair with wrong decimal records.
Recording 5.0 mL or 10.0 mL from a Class A pipet, which drops a legitimate digit. Recording 5.000 mL or 10.000 mL, which invents precision the glass cannot support. Blowing out the pipet and then recording the nominal volume as if nothing changed. Using a pipet with a chipped tip and trusting the two-decimal record anyway. Setting the meniscus without checking eye level, then blaming the glass for a parallax error.
The counterintuitive detail: the most precise-looking record in a notebook, 10.000 mL, is often the least trustworthy entry on the page, because whoever wrote it misunderstood what the tolerance means.
Practical actions and thresholds
Use the following steps to make the decimal decision automatic and defensible.
First, check the pipet's stamp and class. Confirm it is Class A and marked TD. If it is Class B, expect a tolerance near ±0.04 mL, which still supports two decimals for the 10 mL size and puts the 5 mL size at the boundary between one and two.
Second, confirm the reference temperature. If your procedure specifies 20 °C and your room runs warmer, note the deviation rather than adding or removing decimal places.
Third, never blow out a TD pipet. Allow free drainage and touch the tip to the vessel wall. This single habit protects the accuracy your two-decimal record implies.
Fourth, inspect the tip before every use. A chipped or cracked tip is a red flag: retire the pipet rather than record a number it cannot deliver.
Fifth, record to two decimals for both sizes, and carry that format through any dilution calculations so downstream concentration figures inherit a consistent precision.
Sixth, when a procedure demands temperature-corrected volumes, apply the correction and still report to the hundredths place. Reconcile the thresholds explicitly: a ±0.02 mL tolerance maps to two decimals, a ±0.04 mL tolerance maps to one or two depending on context, and a ±0.1 mL tolerance maps to one. Do not let a tolerance of ±0.02 mL coexist with a record of three decimals anywhere in the notebook.
Limits of the rule
The two-decimal convention is a formatting rule tied to a manufacturing tolerance, and it inherits the limits of that tolerance. It tells you how many digits to write; it does not tell you whether the pipet was clean, whether the operator drained it correctly, or whether the solution's viscosity matched the calibration liquid. It also does not settle whether the pipet was calibrated recently or whether the certificate is still valid for your conditions.
Where procedures diverge, follow the governing method or your laboratory's quality system, which may impose its own recording rules for regulated work. The physical reasoning above explains why those rules usually land on the same answer, but the method document is the authority when the two disagree.
The honest bottom line: 5.00 mL and 10.00 mL are the correct records for Class A 5 mL and 10 mL volumetric pipets, because the hundredths digit is the only uncertain digit the glass was built to produce.