Keep measurement units attached to the values you compare
Keep quantity, numerical value and measurement unit connected when comparing or reformatting a report.
About 4 min read · Allow extra time for the practical checks
Identify the quantity and unit behind each number before you compare, shorten or reformat a sentence. A numerical value can change when you express the same quantity in a different unit. Comparing the bare numbers can therefore reverse the meaning of the report.
Follow the assignment or publication’s reporting requirements. This guide concerns ordinary measurement wording and a simple unit-conversion example, not selecting instruments, validating measurements or establishing a statistically meaningful difference.
Read the quantity, number and unit together
Record what was measured, the numerical value and its unit as one connected statement. “The length was 45” is incomplete when the reader cannot establish whether that means millimetres, centimetres or something else. Recover the unit from the checked record rather than choosing the one that makes the result seem plausible.
NIST’s guidance on expressing values of quantities, checked on 2 November 2026, treats a quantity value as a number combined with a unit and emphasises making their relationship clear. It also gives spacing conventions for unit symbols. Use those presentation principles here; the older SP811 guide is not the authority for post-2019 SI definition changes.
For ordinary length values, write a space between number and symbol, as in 45 mm. Do not add a space inside a prefixed unit symbol: mm is one symbol. Check the required style and symbol rather than treating every shortened unit name as interchangeable.
Work through an original fictional comparison
The rods and record below are invented teaching material. No objects were measured, and no instrument, uncertainty or experimental procedure is supplied.
Imagine a fictional record lists rod A’s length as 0.45 m and rod B’s length as 440 mm. A draft says:
Rod B was longer because 440 is greater than 0.45.
That compares numbers expressed in different units. One metre is 1000 millimetres, so 0.45 m corresponds to 450 mm. Expressed in the same unit, the listed values are 450 mm for A and 440 mm for B. Their numerical difference is 10 mm, with A having the larger listed value.
A bounded sentence could say:
The listed lengths were 0.45 m for rod A and 440 mm for rod B. Converting A’s listed value to 450 mm gives a difference of 10 mm between the reported values.
The conversion clarifies the comparison; it does not supply missing evidence about measurement quality. Do not turn that sentence into a claim that a real experiment established a reliable difference, that one instrument was more accurate or that a component meets a design requirement.
| Draft decision | Repair in this fictional exercise |
|---|---|
| Rank 440 and 0.45 directly | Express the listed lengths in a common unit first |
Change 0.45 m to 0.45 mm while standardising labels |
Convert the numerical value as well as changing the symbol |
Report a difference of 10 |
State that the difference between the listed values is 10 mm |
| Add extra decimal places to imply better measurement | Retain only precision justified by the underlying record and task |
Keep the original entries beside your conversion while checking it. Standardising units in prose should not silently overwrite the source record.
Check the sentence against tables and figures
Trace each value to its table heading, axis label, note or original record. A heading may apply a scale factor that a copied cell does not repeat. Make sure the sentence carries the intended unit and scale, not merely the digits you selected.
If you change a display’s unit, check the corresponding numerical values and every sentence that uses them. A revised heading alone does not convert the data. Keep any uncertainty or rounding information connected to the result; unit conversion does not create additional measurement precision.
Read a comparison without its surrounding paragraph as a final check. Can a reader identify the quantity, the two values and the unit that makes their relationship meaningful? Restore the missing information when shortening has removed it.
The equation-writing guide helps explain a calculation and its inputs. The figure-callout guide helps connect a stated comparison to the visual that actually supports it.
