Once your plan is open, you can begin to contribute. However, your annual contribution amount will not be finalized until the end of the year. Final calculations will be done based on your W2 or full year business profit. So, if you have a draft of a funding amount for the year based on your projected numbers, you don't want to make a full contribution. We recommend that you are conservative and contribute a smaller amount (possibly 75%). That way there is some cushion if the annual profit or W2 is lower than expected. The remaining 25% contribution can then be made up to the date the tax return is filed, including extensions.