Many HYIP-projects has calculators of profitableness - where you type sum of deposit and term of investment - and the system will automatically count what profit you receive as a result. Also you can make calculation by yourself. For example you take this plan - 125% after 5 days - that means that 100% is your deposit and 25% is pure profit. It looks like: 125/5=25 and 100/25=4. For 4 days you completely return the deposit and for 5th day go profit. Also keep in mind contribution for input-output that can be described in FAQ-section of HYIP or in Rules.