READY RECKONER CALCULATIONS
Lighting design can be quite an intricate subject and complex computer
programs are available for precise calculation of lighting levels. However,
for many applications, rules of thumb based on practical experience can
be applied to give quick estimates of the number of fittings required for
a scheme.

METHOD
Step 1: Multiply room area (l x w =m2) by lamp factor (shown below)
            to find out ‘total watts’ required.
Step 2: Divide the ‘total watts’ by the wattage of your lamps, to get the
           number of lamps required per room/area.

The following is a table of lamp factors and associated target lighting levels:
Lamp                                 Lamp Factor
                                        Commercial (lux)                              Residential (lux)
12V 50W Dichroic lamps          25 (120)                                            15 (80)
Incandescent lamps                                                                         25 (85)
Fluorescent lamps                    13 (500)
Compact fluorescent lamps       19 (500)
Metal Halide lamps                   8 (500)

NB
Metal Halide is often used for display lighting. In such cases, a target level
of 1000 lux would be more appropriate, i.e. double the number of fittings.
All of the above calculations are purely guidelines based on the most
common situations. Actual levels achieved depend on a variety of factors,
including the luminaire properties, the reflectances of the room surfaces,
the lamp type and the method of use, i.e. direct or indirect lighting.

EXAMPLES
(All examples based on a room 4m x 6m = 24sq.m)
(1) 12V 50W dichroic lamps in a living room (residential)
     Step 1: 24m2 x 15 = 360 Watts Step 2: 360 ÷ 50 = 7.2
     • so use 7 fittings, each with a 50W lamp.
(2) 60W Incandesecent lamps in a living room (residential)
     Step 1: 24m2 x 25 = 600 Watts Step 2: 600 ÷ 60 = 10 lamps
     • so a possible solution would be to use 2 pieces of 3 lamp pendants
     with 4 pieces of single lamp wall brackets.
(3) Fluorescent Modules with category 3 louvres (3 x 18W lamps)
     Step 1: 24m2 x 13 = 312 Watts Step 2: 312 ÷ 54 = 5.78
    • so use 6 3 x 18W modules
(4) Compact Fluorescent lamps e.g. PL fittings with 2 x 26W lamps
    Step 1: 24m2 x 19 = 456W Step 2: 456 ÷ 52 = 8.77
    • so use 9 fittings
(5) 70W Metal Halide lamps
   Step 1: 24m2 x 8 = 192W Step 2: 192 ÷ 70 = 2.74
   • so use 3 fittings (for display purposes 5 fittings would be appropriate)