The best way to find your horses weight, is to use a weightbridge, but it is possible to get an estimate based on measurements of the horses body.
I have found some different formulas in books and magazines, unfortunatly they dont give the same result, but it gives a rough idea of the weight.
Fill in G, B, P and H, then click on the "Calculate Weight" button and you will see the result in the boxes below.

G = girth size in cm. B = length in cm from shoulder to buttock P = length in cm from shoulder to pelvis(hip). H = height in cm.

W1 = kg, Valid for G = (115cm - 237cm)

W2 = kg,

W3 = kg,

W4 = kg, Valid for horses younger than 1.5 year.

W5 = kg, Valid for horses between 1.5 and 2.5 year.

Weight table from the "Horse and Rider" magazine may 2004
Breed	height	girth	weight
Dartmoor pony	112 cm (11 hh)	140 cm	200-250 kg
New Forrest	142 cm (14 hh)	165 cm	350-450 kg
Arab	142 cm (14 hh)	170 cm	400-500 kg
Thoroughbred	163 cm (16 hh)	190 cm	500-550 kg
Warmblood	173 cm (17 hh)	205 cm	580-640 kg
Shire	183 cm (18 hh)	210 cm	700-800 kg

[2.54 cm = 1 inch], [10.16 cm = 4 inches = 1hh (one hand)]