Brick sizes

• Introduction 

• Geographical spread (by volume &thickness) 

• Size variation  –  Width v length    – Thickness v width    –  Thickness v length 

• Manufacturing tolerances 

• Analysis of Lloyd's data   –  Dimensions v date   –  Ratios   –  Geographical spread 

Introduction

Part of the interest of brickwork is the ability to observe subtle (and some not so subtle) differences in size as you walk round different towns. When you 'get your eye in' you will find that different sized bricks, and especially bricks where the sides are in a different ratio, stand out and catch your eye, even before you resort to a ruler.

Several things influenced the size of bricks. They mustn't be too big, or they will be too heavy and awkward to pick up with one hand, while applying mortar with a trowel held in the other. They mustn't be too small, or a wall will need more of them, and more mortar, and more time to lay it. .

In modern times most bricks in UK are made to a standard size of 65x102.5x215mm, and laid with nominal 10mm mortar joints, but in former times the size varied quite a lot.
Some early medieval bricks were as big as 13"x6"x2". Late 15th century bricks were mostly about 9½"x4½"x2". A charter in 1571 specified 9"x4½"x2¼", and in the 18th century, Parliament specified 8½"x4"x2½", which is equivalent to the modern metric brick of 215mm x 102.5mm x 65mm.

What upset the apple cart was a brick tax imposed by the government in 1784. The tax was paid per brick, so brick makers responded by making much larger bricks, fewer of which were needed for a given size wall. An extreme case was Joseph Wilkes  of Measham, who produced bricks double sized the normal size (110x110x235mm) known locally as 'Jumbies' or Wilkes'' Gobs, see right. The government later set an upper limit of 150 cubic inches (10"x5"x3") for a 'brick' – still much larger than bricks had been before the tax.

The tax was repealed in 1850, but by this time, many brick makers, especially in the midlands and north, had moved from hand-made to machine-made bricks. Having invested heavily in machinery, it wasn't easy for them to revert to the smaller sizes, so big bricks persisted. As a result you are likely to find bigger bricks as you move farther north, in buildings built before the introduction of standard modern bricks,.

Geographical spread

I have measured the dimensions of bricks in over 100 different buildings across the country. This wasn't a systematic survey, I just measured any interesting looking bricks whenever I had the chance. I measured the visible faces of what looked like typical bricks in situ, which is less accurate, and may give a smaller measurement than the procedure described in BS EN 771-1, but even allowing for that, the variation between bricks in different buildings is significant.

 
The volume of the bricks I measured ranged from 0.9 to 2.7 litres, compared with 1.4 litres for a standard modern brick. (Allowing for extreme tolerances on all dimensions, the volume of a 'standard' brick could vary between about 1.3 and 1.6 litres.)

The map shows where the bricks were found. The darker blobs show the bigger bricks. For convenience of plotting, the sizes were grouped into five bands, each spanning an interval of 0.4 litres. The map clearly shows the tendency for bigger bricks to be found further north.  

The GB coastline is reproduced from Ordnance Survey map data by permission of the Ordnance Survey  © Crown copyright 2010  

 
The shape of the bricks also tends to vary, with thickness accounting for more of the size difference than length or width. The thickness of the bricks I measured ranged from 42mm to 90mm, a ratio between thickest and thinnest of 2.1 The thickness of a standard modern brick is 65±3mm (ie 62-68mm).

The map shows where the bricks were found, with the darker blobs being the thicker bricks. For plotting, the sizes were grouped into five bands, separated 10mm. The map clearly shows the tendency for thicker bricks to be found further north.  

The GB coastline is reproduced from Ordnance Survey map data by permission of the Ordnance Survey  © Crown copyright 2010

Dimensional variation

The geographic plots are grouped in size bands, and don't show how the individual dimensions relate to each other. The plots below show the actual measurements, plotted against each other in pairs, to illustrate the variation in shape as well as size. In each case, the dimensions of a standard modern brick are shown as a grey line representing the standard dimensions and tolerances.

Width v length
 The bricks I measured had widths ranging from 90­126mm, and lengths ranging from 175­270mm, ie much bigger and smaller than a standard modern brick (102.5 x 225mm).

The two examples from my collection shown here measure 108x230mm and 80x200mm respectively.
 

 
Across the sample, there was quite a strong correlation between width and length, as you would expect, since when laid in a wall, two headers and a layer of mortar should take the same space as a stretcher, ie the width should be just under half the length. The graph shows  this correlation (the dotted line shows a ratio of 2:1).

  Thickness v width 
The bricks I measured had thicknesses ranging from 42-90mm and widths ranging from 90­126mm. These compare with 65mm and 102.5mm for a standard modern brick.
The two examples from my collection shown here measure 86x108mm and 50x80mm respectively.
 

 
There is more variation in thickness than in width, and very little correlation between them, as the graph shows.

Thickness v length
The bricks I measured had lengths ranging from 175-270mm and thicknesses ranging from 42-90mm . These compare with 65mm and 215mm for a standard modern brick.
The two examples from my collection shown here measure 86x230mm and 50x200mm respectively.

 
Thickness and length vary by similar amounts, but the proportional variation in thickness is much greater (a factor of 2.1 between extremes) than in length (a factor of 1.5 between extremes). 

  Manufacturing tolerances

 
Not all variability is due to manufacturers using different sizes. Some is due to variability in the manufacturing process.

In the modern specification, the 'T1' tolerance (the least stringent) allows ±3mm in thickness, ±4mm in width, and ±6mm in length. Historically, much more variation can be found. For example, the pair of bricks shown here were made by the same company, Thomas Lawrence of Bracknell (TLB), and they are almost certainly contemporary, since they were found in the same rubble heap.

  Analysis of Lloyd's data

Nathaniel Lloyd's 1925 book A History of English Brickwork includes the sizes of over 150 different bricks in buildings across England dating from 1050 to 1790, ie 700 years ending just after the brick tax was imposed in 1784. I plotted these values to see whether there was any pattern in these much earlier bricks.

Some of the dates given are exact, but some are more vague. Where Lloyd gave a range, I took the mid value. Where he just specified the century, I used the mid-date, ie xx50. Where it said 'early' or 'late' in the century, I used xx25 or xx75 respectively.

Dimensions – Plotting length, width and thickness shows no consistent variation from around 1430 to 1780.

Length to width – Plotting the ratio of length to width showed no consistent variation with date, being mostly just over 2, as expected.

Width to thickness – Plotting the ratio of width to thickness shows a lot of variability, but a slight downward trend with date, from mostly over 2 around 1450 to mostly under 2 around 1750. In other words, bricks became slightly squarer in cross section over the centuries.

 This plot shows the volume of bricks at different locations, from 0.8 to 2.4 litres, in 0.4 litre bands. As expected for early buildings, most of the samples are in the south east, and there is no obvious pattern to the spread of sizes.

The final diagram plots thickness in 10 mm bands from 40mm to 80mm. Again, there is no obvious pattern in the distribution of sizes.

See also my talk on brickwork .