Easy Centigrade-Fahrenheit and Kilos-Pounds Conversions

Sun, 2007–Nov–25

Traveling between cities in the US and other countries
requires conversions between Centigrade and Fahrenheit, as well
as between Kilos and Pounds. Here's a easy way to do the
calculations, inspired by the faint childhood memories of the
Trachtenberg method of arithmetic manipulations, and achieving
error rates of 1% or less.

Converting Centigrade to Fahrenheit
DSA: DOUBLE, SUBTRACT one-tenth, ADD 32

Converting Fahrenheit to Centigrade
SHA: SUBTRACT 32, HALVE, ADD one-tenth

The exact formula: °F = °C * 9/5 + 32
The easy formula is the same.

Easy conversion using D-S-A (or DSA32) steps,
with example converting 14 °C:
1. Double: 28
2. Subtract one-tenth: 28/10 = 2.8,
rounded to 3, gives 28 - 3 = 25
Ignore negative sign if any, to make it easier, and then
put sign back again.
3. Add 32: 25 + 32 = 57 °F [exact
value is 57.2 °F]

The exact formula: °C = (°F - 32) * 5/9
The easy but approximate formula: °C = (°F - 32) * 1/2 * 1.1

Easy conversion, using S-H-A (or 32SHA) steps,
with example converting 57 °C:
1. Subtract 32: 25
2. Halve: 12.5
3. Add one-tenth: 1.25, rounded to 1.3,
gives 12.5 + 1.3 = 13.8 °C [exact value is 13.888... °C]

If no rounding is done, these steps provide the precise
Fahrenheit value, with no error.

This calculation leaves an error of around 1%; adding
1% to final result will give a more precise value in
Centigrade.
The exact error is (5/9 - 1.1/2) / (1.1/2) = 1.0101...%,
but 99% accuracy should be good enough for most day-to-day
purposes.

Calculator:
°C
→ °F (Exact & Easy)
[Calculators Rounded to 1 Decimal Digit]

Calculator:
°F
→ °C (Exact)
→ °C (Easy)

The Centigrade temperature scale is more properly called
Celsius, but the term Centigrade continues to be in more
common use in many countries.

Kilos and pounds are easier to convert, here are the
examples:

Converting Kilos to
Pounds

Converting Pounds to
Kilos

The exact formula: Pounds = Kilos * 2.2
The easy calculation formula is the same.

The exact formula: Kilos = Pounds / 2.2
The easy but approximate formula: Kilos = Pounds/2 * 0.9

Example converting 160 Pounds:
1. Halve: 80
2. Subtract one-tenth: 8, gives
80 - 8 = 72 [exact value is 72.7272...]

If no rounding is done, this gives the precise value
for Pounds.

This calculation leaves an error of around 1%; adding
1% to final result will give a more precise value in Kilos.
The exact error is (1/2.2 - 0.9/2) / (0.9/2) = 1.0101...%,
but 99% accuracy should be good enough for most day-to-day
purposes.

Easier Method for Temperature - Divide or Multiply by
2?
As to whether the above method for Centigrade-Fahrenheit
conversions is easy - well, it is easier than multiplying by 5
and dividing by 9. On the other hand, this is more complex than
just dividing by 2, which is another way to handle 5 divided by
9, but that results in a greater error, one that is perceivable
when experiencing the cold weather in Montréal!

For Fahrenheit to Centigrade conversions, using 1/2 instead of
5/9 gives an error of 11%, which can be a lot since 86 °F becomes 27 °C
instead of the actual 30 °C.

For Centigrade to Fahrenheit conversions, using 2 instead of
9/5 gives an error that increases in proportion to the size of
the Centigrade value, getting to a maximum of 20%. This can be a
problem, for example, when looking at the thermostat that is set
at 18 °C. Using the double and add 32 method gives 68 °F, but the
correct value is 64.4 °F, and this difference will be noticeable in
getting to the right level of warmth in the room.

This type of thing drove me crackers for years. Even the 'easy' methods weren't that easy. I'm much happier with this. Thank you so much for a useful and informative website.

## Comments

## Temperature/weight conversion methods

This type of thing drove me crackers for years. Even the 'easy' methods weren't that easy. I'm much happier with this. Thank you so much for a useful and informative website.