The FAFSA 4caster is a good tool for estimating your EFC but it’s important to understand its limitations. The big one is this: it only asks for one piece of your income, your AGI. The FAFSA itself requires you to add back some big ticket items that are not included in your AGI. The biggest is your untaxed income, specifically your retirement plan contributions. In a household where both parents work and contribute to retirement savings, this can lead to a big gap between what the 4caster forecasts and the actual EFC calculated by the FAFSA. Two parents who fully funded their 401ks in 2015 would have an additional $36,000 in untaxed income ($48,000 if they are over age 50). In that case, the FAFSA 4caster would underestimate their EFC by almost $17,000 (47% of $36,000).
By all means, use the FAFSA 4caster to estimate your EFC; just be sure to add your pre-tax retirement savings contributions back to your AGI to be more accurate.