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With over 30 years in the builders’ merchant trade, Andrew brings deep knowledge of everything from civils to timber. Now part of the Gilmore team, he helps customers make the right choices with advice built on decades of hands-on experience.
Brick dimensions are defined by BS EN 771-1, the governing European standard for masonry units. Although the nominal UK size of 215 × 102.5 × 65 mm is widely cited, real bricks rarely match these exact figures. Clay shrinkage, kiln behaviour, and allowable tolerance categories result in measurable dimensional variation. When matching new brickwork to existing structures, these variations matter because small differences accumulate over large elevations, affecting coursing, lintel heights and interface details.
For wider matching guidance covering colour, texture and finish, refer to our main brick matching guide at gilmorebuildingsupplies.co.uk/brick-matching-guide.
BS EN 771-1 requires manufacturers to declare a brick’s work size in the order length, width and height. For UK-facing bricks this is typically:
The declared size is a target. Actual finished dimensions vary due to:
BS EN 771-1 Clause 6.2 requires manufacturers to declare the work size and tolerance category of clay masonry units. Compliance with these tolerances must be verified using the reference measurement method defined in BS EN 772-16.
Tolerances define the allowable deviation between the declared work size and the measured mean size of a sample, as well as the permissible range between the smallest and largest unit in that sample. The tables below show commonly published tolerance bands for UK clay facing bricks derived from BS EN 771-1 categories.
| Declared Size (mm) | T2 Lower & Upper Limits (mm) | T2 Tolerances (mm) | T1 Lower & Upper Limits (mm) | T1 Tolerances (mm) | Tm (Manufacturer Declared) |
|---|---|---|---|---|---|
| 215 | 211–219 | ±4 | 209–221 | ±6 | As declared by the manufacturer |
| 102.5 | 99–105 | ±3 | 98–106 | ±4 | |
| 65 | 63–67 | ±2 | 62–68 | ±3 |
| Declared Size (mm) | R2 (mm) | R1 (mm) | Rm (Manufacturer Declared) |
|---|---|---|---|
| 215 | 4 | 9 | As declared by the manufacturer |
| 102.5 | 3 | 6 | |
| 65 | 2 | 5 |
These values align with BS EN 771-1 Annex F tolerance bands when expressed as mean-value limits and range categories for typical UK work sizes. Tm and Rm are manufacturer-declared categories used where a product line does not sit exactly within T1/T2 or R1/R2 classes. All tolerance verification must ultimately be carried out using the reference method in BS EN 772-16.
BS EN 772-16 is the only reference method accepted for confirming compliance with BS EN 771-1 tolerances. It requires controlled conditions and calibrated equipment.
BS EN 772-16 Clause 6 specifies the laboratory reference method for determining dimensional conformity. It mandates controlled environmental conditions and calibrated long-nosed calipers. It is not intended for on-site use.
PAS 70 provides a practical on-site method for measuring brick dimensions during disputes or QA checks. It does not override BS EN 772-16; instead it offers a simplified process suitable for non-laboratory environments.
PAS 70:2013 defines practical on-site dimensional measurement procedures for brickwork, supporting dispute resolution where BS EN 772-16 testing is impractical. PAS 70 does not replace the reference method; it provides a simplified agreed-procedure for site checks. If disagreement persists, BS EN 772-16 laboratory testing is required to determine compliance.
While tolerance categories (T1, T2, Tm) control the deviation of the mean size, range categories (R1, R2, Rm) assess the internal consistency of a batch. Range is the difference between the smallest and largest recorded measurement within the sample of ten bricks:
Range = Largest Measurement − Smallest Measurement
Range values diagnose whether bricks within the same delivery will course consistently or whether material variation may lead to stepped bed joints, inconsistent banding or visible irregularity on long elevations.
For a declared 215 mm brick:
Interpreting this:
This does not mean the bricks are non-compliant. It simply means they meet the manufacturer’s declared range category (R1, R2 or Rm), which determines the expected uniformity of the batch.
Even if the mean dimension complies with T1 or T2 limits, a high range value can result in:
Where range values approach the upper limits of R1, bricklayers should:
R1 and R2 are defined categories in BS EN 771-1 Annex F, while Rm is a manufacturer-declared range value for products that do not align directly with the defined categories. Handmade bricks and certain traditional clay blends typically fall under Rm due to inherent production variation.
BS EN 771-1 Annex F defines internal range categories for assessing dimensional consistency. Range values are calculated using the smallest and largest recorded measurements from the same sample used for mean-value evaluation. Rm is a manufacturer-declared range for products whose natural variation does not align directly with R1 or R2 categories, such as handmade and traditional moulded bricks.
Even 1 mm per course accumulates rapidly:
These differences affect:
Correct dimensional matching should be considered alongside broader assessment factors, documented in the main brick matching guide.
Manufacturers use different clay seams, firing regimes and production machinery. This naturally influences dimensional outcomes. For example:
These differences are not defects. They are inherent to the materials and manufacturing processes and can be beneficial when matching older, non-standard masonry.
BS EN 771-1 permits manufacturer-declared tolerance (Tm) and range (Rm) categories where production methods or clay sources create dimensional behaviours outside standard T1/T2 or R1/R2 bands. These values are not defects; they reflect controlled characteristics of individual product lines and manufacturing sites.
Dimensional variation is normal and permitted under BS EN 771-1. Clay movement during drying, extrusion behaviour and kiln temperature gradients all influence final size. A brick declared as 65 mm may legally measure 63–67 mm (T2) or 62–68 mm (T1). This is why dimensional checks form part of wider brick matching alongside colour and texture assessment.
T1 and T2 are the mean-value tolerance categories defined in BS EN 771-1. T2 allows a tighter dimensional limit than T1. For example, a declared 65 mm brick must fall within 63–67 mm (T2) or 62–68 mm (T1). These values apply to the mean of ten sampled bricks measured to BS EN 772-16 or PAS 70 depending on context.
Range values assess consistency within a sample. R1 and R2 specify the maximum permitted difference between the largest and smallest brick measured. For a 215 mm brick, R1 allows up to 9 mm of variation, while R2 allows up to 4 mm. Rm indicates a manufacturer-declared range outside standard classifications, which is common for handmade bricks.
PAS 70 is the accepted on-site procedure. Ten bricks are selected from multiple packs. Length is recorded against a datum edge, width is taken using a rule or calipers and height is calculated by averaging two readings, one from each end. All results are rounded to the nearest millimetre and compared with the declared T and R categories.
BS EN 772-16 is a laboratory method using calibrated equipment under controlled conditions. PAS 70 is designed for practical site-based checks where laboratory tools are not available. PAS 70 results may be used for dispute resolution provided all parties agree. If not, BS EN 772-16 testing must be carried out at a factory or accredited laboratory.
Small differences accumulate over multiple courses. Even 1 mm per course becomes noticeable across a full storey. Poor dimensional alignment affects tie-in points on existing walls, window head levels, lintel seating and DPC alignment. Accurate dimensional matching is essential for extensions and repairs, especially when using facing bricks intended for visible work.
Yes. Handmade bricks commonly use manufacturer-declared tolerances (Tm and Rm) because of the inherent variability in hand forming and drying processes. This variation is often desirable when matching older masonry with irregular coursing patterns. It does not indicate lower quality. Many premium handmades are intentionally produced with wider dimensional behaviour for authenticity.
Yes. Mean-value tolerances and range tolerances are independent. A product may meet its declared mean-value tolerance but still show substantial spread within the sample if the product is correctly declared under R1 or Rm. This is why both criteria must be reviewed when checking compatibility for extensions, refurbishments and detailing work.
Please note: The information provided on this website is for general guidance only and should not be relied upon as professional advice.
Building methods, material specifications, and regulations can vary depending on location, project design, and site conditions.
Always refer to the latest Building Regulations, manufacturer data sheets, and consult with a qualified structural engineer, surveyor, or building control officer before starting any construction work or making design decisions.
Gilmore Building Supplies accepts no responsibility for loss, damage, or injury resulting from reliance on the information provided.
