Here's a formula for 7 pt:
The following formula is one of Jackson and Pollock's that has been used widely in research:
BD= 1.11200000 - 0.00043499(X) + 0.00000055(X)(X) - 0.00028826 (A)
X = Sum of chest, axilla, triceps, subscapular, abdomen, suprailium, and thigh skin folds (mm).
A = Age in years.
This formula had a correlation of .915 when compared to hydrostatic weighing to estimate body density. The standard error of estimate was .008 expressed in body density and a standard error of estimate of 3.5 when expressed in % BF using the following equation by Siri (1961):
% Fat = [(4.95/BD) + 4.5] 100