Start with the wall you actually have
A room dimension on an old floor plan is a useful starting point, but it is not a cutting measurement. Measure the finished surface where the decorative work will sit. Keep a sketch with the wall name, the unit you are using and one clearly marked reference corner. Take width measurements near the bottom, middle and top of the intended area. If they differ, keep all three numbers. A rectangular drawing cannot make a wall square.
Separate the wall from the panel area
Write down the full wall width and height, then the space you want to leave at the left and right edges, above the floor and below the ceiling. Baseboards, coving and existing rails may occupy part of that space. In our wall-panel calculator, the top and bottom clearances are measured from the full wall boundary to the outside edge of the nearest frame. The side margin works the same way. Do not subtract the baseboard once from the height and then again as a bottom clearance.
Record obstacles from one origin
Measure a door, window or other obstacle from the top-left corner of the same wall. Record its left position, top position, full width and full height. Include the existing trim if it must remain clear. An outlet is a physical obstruction, not an invitation to move wiring: use the planner to identify a clash and review the installation separately. The calculator can check one rectangular obstacle against the planned moulding strips; it does not relocate frames automatically.
Try a complete example
For a 300 cm wide wall with 20 cm margins, three columns and 15 cm gaps, the width available to the frames is 300 − 40 − 30 = 230 cm. Each outside frame width is 230 ÷ 3 = 76.67 cm. If the wall is 240 cm high and the top and bottom clearances are both 25 cm, one row has an outside height of 190 cm. These are design dimensions, before cutting allowances.
Check the design at full size
Mark the planned outside edges with a removable method suitable for your finish, or compare a paper sketch against the real wall. Look at door movement, furniture, existing trim and visual alignment. Re-measure before cutting. The downloaded diagram is a scaled planning illustration, not a full-size drilling or installation template. If the surface is uneven, the final fit may require adjustments that a rectangular calculator cannot predict.