Determine the original value of the asset. This can be supported with a receipt or brokerage statement. Let's say you purchased $1,000 worth of XYZ stock on Week 1.

Determine the ending value of the asset. This is the market value of the asset at the beginning of Week 2. Let's say the value of the stock at the beginning of Week 2 is $1,200 and $1,500 at the end of Week 3.

Subtract the ending or current value from the original value. For instance, $1,200 - $1,000 = $200 and $1,500 - $1,200 = $300.

Divide the difference by the original value. To calculate the return from Week 1 to Week 2, divide the difference between Week 1 and Week 2 by the previous week. For instance the weekly return for Week 1 to Week 2 is $200/$1000 or 20 percent (.2 x 100). The return from Week 2 to Week 3 is $300/$1200 or 25 percent (.25 x 100).