Activity – Conversion between metric and imperial units of weight
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We will start by revising metric units of weight and then move on to useful conversions between metric and imperial measures.
For metric units, grams (g) are the key measure, with other larger units, kilograms (kg) and tonnes (t) and a smaller unit, milligrams (mg).
They are related by
Convert: (a) 1300 mg to g (b) 2.75 g to mg (c) 1.25 kg to g
(d) 740 g to kg (e) 1.4 tonnes to kg (f) 1250 kg to tonnes
(a) To convert milligrams to grams, we divide by 1000 to give:
1300 mg ÷ 1000 = 1.3 g
(b) To convert grams to milligrams, we multiply by 1000 to give
2.75 g x 1000 = 2750 mg
(c) To convert kilograms to grams, we multiply by 1000 to give
1.25 kg x 1000 = 1250 g
(d) To convert grams to kilograms, we divide by 1000 to give
740 g x 1000 = 0.740 kg
(e) To convert tonnes to kilograms, we multiply by 1000 to give
1.4 t x 1000 = 1400 g
(f) To convert kilograms to tonnes, we divide by 1000 to give
1250 kg ÷ 1000 = 1.25 t
Complete this table of weights by adding in the missing weights in each row.
mg g kg t 0.25 25 1500 4500
The imperial units for weights are ounce (oz), pound (lb) and stone (st).
These are related by
16 ounces (oz) = 1 pound (lb)
14 pounds (lb) = 1 stone (st)
and it follows that
I stone = 224 ounces
Convert (a) 4 lbs to oz (b) 80 oz to lbs (c) 2.5 st to lbs (d) 84 lbs to st
(a) To convert pounds (lbs) to ounces (oz), we multiply by 16 to give
4 lb x 16 = 64 oz
(b) To convert ounces (oz) to pounds (lbs), we divide by 16 to give
80 oz ÷ 16 = 5 lbs
(c) To convert stones (st) to pounds (lbs), we multiply by 14 to give
2.5 st x 14 = 35 lbs
(d) To convert pounds (lb) to stones (st), we divide by 14 to give
84 lbs ÷ 14 = 6 st
Complete this table by adding in the missing weights.
ounces (oz) pounds (ibs) stones (st) 448 56 1.5
We can also convert between metric and imperial units using these approximations:
We can use these approximations to convert, for example, 6 kg to lbs as
6 kg ≈ 6 x 2.2 lbs = 13.2 lbs
Note that we use the symbol ≈ to mean ‘approximately equal’
Similarly, we can convert 8 oz to g as
8 oz ≈ 8 x 28.35 g = 226.8 g
Complete these conversion tables, using the approximations given above.
Round to 2 decimal places where needed.
kg lbs 57 137 4.5 g oz 450 17 2500
Body mass index (BMI) is a value calculated from the mass and height of an adult.
It is given by the formula
where W is weight in kilograms and h is the height in metres of the person.
We can calculate the BMI given the values of W and h.
For example, if someone weighs 56 kg and has height 1.45 m, then their BMI is given by:
BMI = = 26.6 (to one decimal place)
The BMI values give an indication of the balance of weight and height. The table below gives an indication of a person’s health.
So our person with weight 56 kg and height 1.45 m is slightly overweight.
Calculate the BMI (to one decimal place) for each of the following people:
(a) weight 45.6 kg and height 1.52 m BM! =
(b) weight 56 kg and height 1.65 m BM! =
(c) weight 13 st 4 lbs and height 5 ft BM! =
You will need to convert values to metric units; note that 1 foot is approximately