I've heard there are online programs. You need to know your house construction pretty well to get an accurate calculation.

Below is an example of how one exterior wall would be rated. Then you also figure windows and doors after that.

Walls: (Each exterior wall, on each floor, must be done individually)
Any windows and doors in a wall must be included with that particular wall. A rough drawing of each wall can be supplied, if easier, showing windows and doors. All measurements, in feet and inches for walls, windows, and doors, must be included for the length, width, or height.

What direction does the wall face?
O North
O Northeast
O Northwest
O East
O East Northeast
O East Southeast
O West
O West Northwest
O West Southwest
O South
O Southeast
O Southwest

O A: Wood frame, with sheathing, siding or brick
Wall cavity
O None
O R-11 3 ½"
O R-13 4"
O R-19 5 ½"
O R-27 7 ½"
O R-30 8 ½"
O R-33 9-10"

Select exterior insulation
O None
O R-1.3 ½" Asphalt board
O R-1.8 ½" Beadboard
O R-2.5 ½" Extruded poly board
O R-2.7 ¾" Beadboard
O R-3.6 1 in. Beadboard
O R-3.8 ¾" Extruded poly board
O R-5.0 1" Extruded poly board
O R-8 Sheathing

O B: Masonry, above grade
Select insulation value
O R-5 1"
O R-11 3 ½"
O R-19 5 ½"

Wall construction  no added insulation
O 8" or 12" block
O 8" block and 4" brick

O C: ICF Insulated concrete form, above grade
Select ASTM R-value
O R-12 to R-14
O R-14 to R-16
O R-16 to R-18
O R-18 to R-20
O R-20 to R-22
O R-22 to R-26
O R-26 to R-30
O R-30 to R-36

O D: SIP Structural Insulated Panels
O 4 ½" panel
O 6 ½" panel

Exterior finish
O Stuco or siding
O Brick
O Split logs on both sides

O E: Logs

Circle the log thickness: 6" 7" 8" 9" 10" 11" 12"

O F: Block or brick, extends to 5Â below grade
Select insulation value
O R-5 1"
O R-11 3 ½"
O R-19 5 ½"

Wall construction  no added insulation
O 8 or 12 inch block

O G: Block or brick, extends over 5Â below grade
Select insulation value
O R-5 1"
O R-11 3 ½"
O R-19 5 ½"

Wall construction  no added insulation
O 8 or 12 inch block

O H: ICF, extends to 5Â below grade
Select ASTM R-value
O R-12 to R-14
O R-14 to R-16
O R-16 to R-18
O R-18 to R-20
O R-20 to R-22
O R-22 to R-26
O R-26 to R-30
O R-30 to R-36

O I: ICF, extends over 5Â below grade
Select ASTM R-value
O R-12 to R-14
O R-14 to R-16
O R-16 to R-18
O R-18 to R-20
O R-20 to R-22
O R-22 to R-26
O R-26 to R-30
O R-30 to R-36

Don't use the on-line programs. Instead, use programs that run on your computer that you can obtain on-line.

HVAC-Calc at hvac-calc.com is good.

HEED, linked below, is better and free, but requires more detailed input.

I have used both and recommend both.

Here is a link that might be useful: HEED

The excerpt from above is from hvac-calc. It is my questionnaire version for homeowners.

Alot of the brain work is in the program.

One of the biggest problems is accounting for infiltration loss/gain.

Without a blower door test it is a guess at best, and for an older house can be a major source of loss/gain.

Whatever number the program spits out for heat load, you add 25% to it for sizing a furnace, as a buffer. With A/C, you add nothing because you want to dry the air.

It's better than the old method of looking at the house from across the street and holding your hand out and counting fingers.