How to calculate heat load
The 5-step tutorial

Calculating a heat load is necessary before the installation of a radiant heating system can begin, since different types of radiant heating systems have different BTU output values.
A typical heat load calculation consists of surface heat loss calculation and heat loss due to air infiltration. Both should be done separately for every room in the house, so having a floor plan with dimensions of all walls, floors, ceiling, as well as doors and windows is a good place to start.

Below is a sample 5-step manual to surface heat loss calculation:

Step 1 – Calculate Delta T (Design Temperature):

Delta T is a difference between indoor design temperature (T1) and outdoor design temperature (T2), where indoor design temperature is typically 68-72F depending on your preference, and outdoor design temperature is a typical low during the heating season. The former can be obtained by calling your local utility company.
Assuming that T1 is 72F and T2 is –5F, Delta T = 72F - (-5F) = 72F + 5F = 77F

Step 2 – Calculate surface area:

If the calculation is done for an outside wall, with windows and doors, the calculations for the window and door heat loss should be done separately.

Wall Area = Height x Width - Door Surface - Window Surface
Wall Area = 8ft x 22ft - 24sq ft - 14sq ft = 176sq ft - 38sq ft = 138sq ft

Step 3 – Calculate U-value:

Use "Typical R-values and U-values" guide to obtain the wall R-value.

U-value = 1 / R-value
U-value = 1 / 14.3 = 0.07

Step 4 – Calculate wall surface heat loss:

Surface heat loss can be calculated using the formula below:

Surface Heat Loss = U-value x Wall Area x Delta T
Surface Heat Loss = 0.07 x 138sq ft x 77F = 744 BTUH
(U-value is based on assuming a 2x4 wood frame wall with 3.5" fiberglass insulation)

Step 5 – Calculate total wall heat loss:

Follow the steps 1 through 4 to calculate heat loss separately for windows, doors, and ceiling.
Door Heat Loss = 0.49 x 24sq ft x 77F = 906 BTUH
(U-value is based on assuming a solid wood door)
Window Heat Loss = 0.65 x 14sq ft x 77F = 701 BTUH
(U-value is based on assuming a double-panel window)
Ceiling Heat Loss = 0.05 x 352sq ft x 77F = 1355 BTUH
(U-value is based on assuming a 6" fiberglass insulation. Ceiling surface is 22ft x 16ft)

Now, add all the number together:
Total Wall heat loss = Wall loss + Window Loss + Door loss + Ceiling loss
Total Wall heat loss = 744 BTUH + 906 BTUH + 701 BTUH + 1352 BTUH = 3703 BTUH

Air infiltration rates should always be taken into consideration.
The following formula can be used to calculate heat loss for a room due to air infiltration:

Air Infiltration Heat Loss = Room Volume x Delta T x Air Changes per Hour x 0.018
Where Room Volume = Length x Width x Height

Air Changes per Hour accounts for air leakage into the room.
For example: Air Infiltration Heat Loss = (22ft x 16ft x 8ft) x 77F x 1.2 x 0.018 = 4683 BTUH

For actual calculations, contact your contractor or system designer.