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Check the terms and conditions of the savings bond to find the annual interest rate. For paper bonds, this information comes with the bond. For electronic bonds, you will need to go to TreasuryDirect.gov and log onto the account the bond was purchased under.

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Calculate the monthly interest rate. U.S. Savings Bonds are compounded monthly, so divide the annual percentage rate by 12 to get the monthly rate.

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Find the purchase price of the bond. For electronic bonds, this is the face value. The interest is simply added to the face value. Paper bonds are sold at a discount from the face value. For example, if the face value is $50, the purchase price might be $25. The discounted price is always set so that the bond reaches the face value at its maturity date.

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Multiply the monthly interest rate by the purchase price (starting value) of the bond. Add the interest to the starting value to find the value of the bond at the end of one month. Then, using the new value as the starting value, repeat this calculation for the number of months since the bond was purchased.

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Simplify Step 4 by using a financial calculator. The formula to enter is C = S*(1+ I)^M. S is the starting (purchase) value of the bond, I is the monthly interest rate, and M is the number of months since the bond was purchased. C is the current value of the bond you are calculating.

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Make things even simpler by using the TreasuryDirect.gov online calculator. The calculator can link to your account and find the value of your U.S. Savings Bonds as well as performing a number of other calculations.