The water capacity of a boiler can be calculated from the boiler's power output (BTU), which is dependent on house size and insulation. Engineers normally use a typical formula to calculate the BTU and water capacity needed for a boiler but this is not an accurate estimate. Standard formulas normally calculate a water output too powerful for the house or, in the worse case, too low. An inaccurate estimation of boiler water capacity will result in boiler inefficiently and wasted resources.
Things You'll Need
 Measuring tape
 Calculator

Measure each room of your house in cubic feet. Use a measuring tape to calculate the width, length and height of each room and multiply these figures together. Note down all the room sizes separately.
If your manufacturer has left the individual power output for your radiators you can also add those figures together rather than measuring each room. You can skip to step 5 if you have the power output for your radiators.

Calculate the estimated BTU by noting down each room type, the amount of windows and type of windows in each room. Measurements for lounges and dining rooms should be multiplied by 5, bedrooms by 4 and kitchen areas by 3. Add 15% of the total to each calculation for rooms facing north, add 20% for rooms with French windows and deduct 10% for rooms with double gazed windows.

Add the individual room calculations together. This will give you the approximate BTU boiler output for the house rather than a standard engineer BTU output.

Divide the BTU by 10,000 and multiple by 4. This will calculate the estimated water capacity in gallons for your boiler  for example, a home with a 50,000 BTU output would require 20 gallons of water.
Tips & Warnings
 You can use online BTU calculators (see resources) which will allow you to add other factors to the calculation (such as wall insulation and the average temperature of your area).
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References
 Photo Credit white radiator image by Ekaterina Sidorenko from Fotolia.com