Conversion Table:
Metres

Feet

deg C

deg F

kW/h

BTU/hr


1 Metre

x

3.2808399

x

x

x

x

1 Foot

0.3048

x

x

x

x

x

1 deg C

x

x

x

33

x

x

1 deg F

x

x

x

17

x

x

1 kW/h

x

x

x

x

x

3412

1 BTU/hr

x

x

x

x

0.0002928

x

The first port of call when choosing a portable heater for an area is to size the area. The size is chosen in three dimensions, so must include length, width and height. The area being heated must also be enclosed. The calculation for heat requirement is done in cubic metres so you require the length x width x height in metres. If you only have the dimensions in feet, then these will need to be multiplied by 0.3048 each in turn to give dimensions in metres, then those figures must be calculated to get the volume in cubic metres.
Once you have the volume in cubic metres, you need to decide on a temperature rise. Choosing a required temperature is not possible, as the amount of heat required is based on the insulation value and temperature rise  it will obviously take less heat energy to raise a room from 15 deg C to 20 deg C than it would to raise a rom from freezing (0 deg C) to 10 deg C.
Having chosen a temperature rise, the final part of the calculation is the K value or insulation index. If the room is well insulated, on a similar level to a house, then a K value of 1.2 is correct. If it's insulated, but less so, like a portacabin, caravan, or workshop then 2.2 is the correct K value. For draughty workshops, open warehouses and uninsulated buildings then use 3.0.
If there is no insulation at all, for example a marquee or a building site, then the value 4.0 should be used for K value.
So the full equation is (length x width x height) x temperature rise in deg C x 4 x K Value = BTU/hr required.
The BTU/hr required can be divided by 3412 to give kW/hr.
So for example a 30ft x 60ft marquee with a 4 m high ceiling, requiring to be at 22 deg C when it's 5 deg C outside:
30 x 0.3048 = 9.144 ( 30ft in Metres.) x 2 = 18.288 (60ft in Metres.)
9.144 x 18.288 x 4 = 668.90188 (Volume in cubic metres.)
668.90188 x 27 (total deg C temperature rise.) = 18060.35
18060.35 x 4 = 72241.4 x 4.0 (K value for Marquee (uninsulated)) = 288,965.6 BTU/hr
288,965.6 BTU/hr divided by 3412 = 84.690973 kW/hr output.
When choosing a heater, I would go for the one above the actual requirement, rather than the one below to cover yourself.