Under RICS NRM 2, you round each dimension to the nearest 10 mm, then present the resulting quantity to the nearest whole unit. Tonnes are given to two decimal places, and a quantity of less than one unit is given as one unit. Part 3 of the rules can set a different precision for a particular work section, so the tabulated rule for the item always has the final say.
This article sets out those rules in the order you apply them. It gives worked examples, shows where rounding errors tend to creep into a take-off, and finishes with a short checklist for keeping a Bill of Quantities consistent from one estimator to the next.
TL;DR
- Round every dimension to the nearest 10 mm before you calculate: 5 mm and over counts as 10 mm, under 5 mm is disregarded.
- Present ordinary quantities to the nearest whole unit, tonnes to two decimal places, and anything under one unit as one unit.
- Work in metric: mm, m, m², m³, nr and t, as the relevant Part 3 table specifies.
- A work-section rule in Part 3 overrides the general position, so check the table before you finalise a quantity.
- Write down your rounding method once and apply it to the whole job, so the BoQ can be checked and reproduced.
Table of Contents
- What NRM2 means for rounding
- Rule 1: dimensions to the nearest 10 mm
- Rule 2: quantities to whole units, tonnes to two decimals
- Order of operations: round the dimension first
- Worked examples
- Where Part 3 rules change the picture
- Keeping rounding consistent across a take-off
- Common rounding mistakes
- NRM2 compared with SMM7
- Where a drawing-reading tool fits
- FAQ
- Sources
What NRM2 means for rounding
NRM 2 is the RICS standard titled Detailed measurement for building works. RICS describes it as providing standard rules for measuring and describing building work and for preparing Bills of Quantities and quantified schedules. The second edition took effect on 1 December 2021. RICS reissued the NRM documents as practice information in October 2022, with no material content changes from the 2021 edition.
The rounding conventions sit in the general rules at section 3.2.1, before the tabulated work sections. They matter because a Bill of Quantities is a contractual tool. RICS defines it as a list of building components with identifying descriptions and quantities, and once it is contractually adopted it supports tendering, interim-payment valuation and the valuation of varied work. A quantity that has been rounded inconsistently can therefore reappear later, as a valuation query or a dispute about a variation.
If you want the wider picture of the standard first, our guide to NRM2 explained covers structure and purpose. This article stays with the numbers.
Rule 1: dimensions to the nearest 10 mm
Section 3.2.1 of NRM 2 says dimensions are taken to the nearest 10 mm. A dimension with 5 mm or more beyond the 10 mm step is treated as 10 mm, and a dimension with less than 5 mm beyond it is disregarded.
In practice, take the dimension in millimetres and look at the last digit:
| Measured dimension | Last digit | Rounded to nearest 10 mm |
|---|---|---|
| 5,234 mm | 4 (under 5) | 5,230 mm |
| 5,235 mm | 5 (5 or over) | 5,240 mm |
| 2,498 mm | 8 | 2,500 mm |
| 3,004 mm | 4 | 3,000 mm |
| 0.917 m | 7 | 0.92 m |
Working in metres, the rule means every dimension carries two decimal places. A figure such as 2.436 m is written as 2.44 m, and 2.434 m becomes 2.43 m.
The point of the rule is that the BoQ is not a record of survey precision. Nobody sets out a stud wall or a ground-floor slab to the nearest millimetre on a drawing, and the 10 mm step keeps dimension columns readable and repeatable between two people measuring the same drawing.
Rule 2: quantities to whole units, tonnes to two decimals
The second part of the convention covers how the calculated quantity is presented. Generally, NRM 2 gives quantities to the nearest whole unit. The exceptions in section 3.2.1 are:
- Tonnes: given to two decimal places.
- Quantities of less than one unit: given as one unit.
NRM 2 measurement symbols include mm, m, m², m³, nr (number) and t (tonne). The standard's symbols list confirms a metric basis for ordinary building-work measurement.
| Item | Unit | How the quantity is presented |
|---|---|---|
| Blockwork wall | m² | Whole m² |
| Concrete foundation | m³ | Whole m³, or 1 m³ where under one |
| Pipe or kerb run | m | Whole m |
| Doors, sockets, rainwater outlets | nr | Whole number |
| Structural steel | t | Two decimal places |
The qualifier is always the same: "subject to any specific work-section rule". Part 3 of the standard sets the quantities and descriptions for measured work in tabulated form, so where a table states its own unit or a different precision, follow the table.
Order of operations: round the dimension first
The sequence is the part most often got wrong. Apply the dimension rule before you calculate the measured quantity, and apply the quantity rule to the result.
- Read the dimension from the drawing or scale.
- Round it to the nearest 10 mm.
- Multiply the rounded dimensions to get the quantity.
- Present that quantity to the applicable precision: whole unit, two-decimal tonnes, or one unit where under one.
- Check the relevant Part 3 table for a specific rule that overrides steps 2 to 4.
Rounding only at the end, from raw dimensions, can produce a different number from rounding as you go. Most of the time the difference vanishes in the whole-unit rounding. Near a boundary it does not, as the next section shows.
Worked examples
The figures below are invented to illustrate the arithmetic. They are not taken from a real project.
Example 1: a floor area near a boundary
A room measures 3.115 m by 4.005 m on the drawing.
| Method | Dimensions used | Calculation | Quantity presented |
|---|---|---|---|
| Round dimensions first (NRM 2 sequence) | 3.12 m × 4.01 m | 12.5112 m² | 13 m² |
| Calculate from raw dimensions | 3.115 m × 4.005 m | 12.4756 m² | 12 m² |
The two methods differ by a whole square metre. Applying the dimension rule first gives 13 m². On one room this is trivial, but repeated across a set of floors and finishes, a mixed method gives you a BoQ that cannot be reconciled with the take-off sheets.
Example 2: a small volume
A pad foundation works out at 0.38 m³ after rounded dimensions are multiplied. The quantity is under one unit, so it is presented as 1 m³. The quantity rule for sub-one quantities exists so that a small item is still priced and does not drop out of the document.
Example 3: tonnage
Steel sections add up to 1.2345 t. Tonnes are given to two decimal places, so the quantity is 1.23 t. A figure of 1.2351 t would be 1.24 t. Because tonnes keep their decimals, steelwork is the item where rounding discipline shows most clearly in the price, which is why a separate structural steel take-off method is worth setting up.
Example 4: lengths and numbers
A run of kerb is built from segments of 4.996 m, 3.504 m and 7.251 m. Rounding each gives 5.00 m, 3.50 m and 7.25 m, a total of 15.75 m, which is presented as 16 m. Enumerated items such as doors are counted, so they are already whole numbers and rounding does not apply.
Where Part 3 rules change the picture
The general rule is only the starting point. Each work section in Part 3 sets out what is measured, in what unit, and with what descriptive detail. Some items are measured as extra over, some in linear metres rather than area, and some carry rules about what is included or deducted. Excavation is a good example of a section with its own set of conditions, and preliminaries follow a different approach again, which our note on requiring contractor breakdowns for NRM2 preliminaries explains.
A reliable habit is to read the table row for each item you measure and ask three questions:
- What is the measurement unit?
- Does the row state a precision or minimum different from the general rule?
- Which deductions and inclusions does the rule define, so that you do not double count?
Our NRM2 take-off rules summary lists the checks that make a BoQ easier to audit, and fits well beside this one.
Keeping rounding consistent across a take-off
The rules are short. The failures come from habit and tooling. Spreadsheet cells show two decimals but store many, a measuring tool returns figures to the nearest millimetre, and a colleague rounds the total instead of the dimensions. These practices keep the document clean:
- Fix one method for the project. Record in the take-off notes that dimensions are rounded to the nearest 10 mm before multiplication and quantities presented to the NRM 2 precision.
- Round in the formula, not the display. In Excel, a cell formatted to two decimals is not rounded. Use a rounding function on each dimension so the stored value is the value you present.
- Round at the right level. Round dimensions, then the quantity. Do not round subtotals and then add them, or round a total and then apply it to another calculation.
- Mark the exceptions. Where a Part 3 rule changes the precision or unit, note the clause reference beside the line.
- Keep the sheet traceable. A second person should be able to take your dimension column and reproduce your quantity.
- Sample-check near boundaries. Re-run any line where the unrounded quantity sits close to a half unit, as in Example 1.
If you also set tolerances for scale or drawing error, keep them separate from rounding. A tolerance describes how far you trust the measurement, and rounding describes how you present it. Our post on measurement tolerances in UK take-offs goes through seven QC checks for that, and the guide to practical NRM2 measurement mistakes lists the slips that show up most in review.
Common rounding mistakes
- Rounding the area, not the sides. Multiplying raw dimensions and rounding at the end skips the 10 mm step.
- Treating a 4 mm remainder as rounding up. Under 5 mm beyond the step is disregarded.
- Reporting tonnes as whole numbers. Steel and reinforcement usually need two decimals.
- Dropping small items. A quantity under one unit is presented as one unit.
- Showing decimals on whole-unit items. Cells that display 12.48 m² while pricing from 12 m² create an audit gap.
- Mixing methods between sections. One estimator rounds as they go, another at the end, and the totals will not reconcile.
The same discipline carries into specialist packages. The steps in MEP quantity take-off for NRM2 and in the house extension take-off checks show how a consistent method plays out on real building types.
NRM2 compared with SMM7
NRM 2 replaced the Standard Method of Measurement for Building Works. The first edition of NRM 2, effective from 1 January 2013, states that the latest edition of that earlier method was SMM7, published in 1988. RICS reported in November 2021 that SMM7 had been superseded, although some client consultants kept using it and some contractors adapted NRM 2 with SMM7 elements.
That last point matters for rounding. If you receive a BoQ or a subcontractor return built on a blended method, do not assume it follows the NRM 2 conventions. Ask which rules were used, and re-measure or restate the quantities if you need them on the NRM 2 basis. RICS also says earlier NRM editions are out of date and should not be used on new projects, although they can be downloaded for reference, so make sure you work from the current version of the standard.
Where a drawing-reading tool fits
Rounding rules are easiest to apply when the dimension column is visible and every number can be traced back to the drawing. That is the idea behind QuantiFlow, which is in development. It reads construction drawings and produces a draft Bill of Quantities for a quantity surveyor to review and sign off, and the rounding, the Part 3 rule checks and the final quantities remain your professional decisions.
The published plans are Solo at £69 per month and Business at £299 per month, with an Enterprise tier for larger teams. You can see the options on the QuantiFlow site. If you are weighing up different approaches first, our overview of estimating software for UK SMEs and the guide to measuring PDF drawings for NRM2 take-offs set out what to look for.
FAQ
What is the NRM2 rounding rule for dimensions?
Dimensions are taken to the nearest 10 mm. A remainder of 5 mm or more counts as 10 mm, and a remainder under 5 mm is disregarded. So 5,235 mm becomes 5,240 mm and 5,234 mm becomes 5,230 mm. The source is section 3.2.1 of RICS NRM 2.
Do I round the dimensions or the final quantity?
Both, in order. Round each dimension to the nearest 10 mm, calculate the quantity from those rounded dimensions, then present the quantity to the applicable precision. Normally that is a whole unit, with tonnes to two decimal places. A specific Part 3 rule may say otherwise.
How are quantities under one unit shown in an NRM2 BoQ?
A quantity of less than one unit is given as one unit. A 0.38 m³ item is therefore presented as 1 m³. This stops small items being omitted from the priced document.
Does NRM2 use metric units?
Yes. The measurement symbols include mm, m, m², m³, nr for enumerated items and t for tonnes. The unit for each item is set by the relevant work-section table in Part 3.
Is NRM2 rounding the same as SMM7?
Do not assume it is. NRM 2 replaced SMM7, and some projects still use blended methods. If a BoQ was prepared on SMM7 or an adapted basis, check which rules it used before comparing quantities with an NRM 2 take-off.
Sources
- RICS New rules of measurement: NRM 2, Detailed measurement for building works, 2nd edition
- RICS NRM: New Rules of Measurement
- RICS NRM 2: Detailed measurement for building works, 1st edition
- RICS Construction Journal: NRM update launched
Recommended
- NRM2 takeoff rules to make BoQs audit ready for UK quantity surveyors
- NRM2 explained: the RICS standard for measuring building works
- NRM2 Trade Boundaries: Where One Work Section Ends
- NRM2 Item Coding: How Bill of Quantities Codes Are Built
This article is for general information only and is not professional, legal or commercial advice. Quantity surveying decisions should be taken by a qualified professional with reference to the specific project, drawings and contract in question. Content is produced with AI assistance and reviewed before publication. QuantiFlow Ltd accepts no liability for reliance on it.