Construction NumeracyCalculating Ratios and Proportions

Stonemasonry Department 2012

Perimeter

‘a ratio represents, simply, for every amount of one thing, how much there is

of another thing’

Wikipedia 2012

Ratios

Ratios are used by all trades in the construction industry. One of the most common uses for ratios is when specifying a mortar mix. For example the image above shows that in order to mix

the specified mortar 5 buckets of sand and 2 buckets of lime are used. This is known as a 5:2 mix.

RatiosCan you identify the ratio of sand to lime in the mortar mixes

shown below?

= 4:1

= 3:1

= 3:2

= 4:2

RatiosIn the last example on the previous slide we established a ratio of 4:2. When using ratios we normally try to reduce them to

their simplest form. This means that rather than saying 4:2 we divide each side by 2 and get a ratio of 2:1.

:

:

RatiosWhich of the ratios shown below is the odd one out?

That’s right the 5:2 is the odd one out as all the others are variables of 3:1.

RatiosIt is not only mortar mixes that are shown as ratios. In the construction industry workers are often placed into small

squads so that they are able to work efficiently. An example of this is the bricklayers 2 and 1 squad where one labourer is

assigned to work with 2 bricklayers.

:

RatiosIn previous examples we simplified the ratios to make them

more easy to understand. We can however also multiply ratios to calculate how many of one value we need in proportion to

another value. Using a ratio of 2:1 can you calculate how many labourers are required to service 40 bricklayers?

: x20

RatiosWe can also have ratios which show the value of three or more different things. We have now added some colouring dye to our mortar mix from earlier. Can you guess how the ratio might be

displayed?

5 : 2 : 1The mix is now referred to as a 5:2:1 mix as there are five parts

sand to two parts lime to one part colouring dye.

Developed by The Stonemasonry DepartmentCity of Glasgow College

2012