Savings goal calculator

Your pot does two things at once. What you already have grows on its own, and each new monthly contribution starts growing from the month you make it. The calculator compounds both, month by month, with contributions treated as landing at the end of each month.

To find the required contribution it runs that in reverse: it grows your existing savings forward to your deadline, sees what is still missing, and works out the monthly amount whose own growth exactly fills the gap. That is why the answer is not simply the shortfall divided by the number of months: the growth on your contributions is doing part of the work for you.

The required contribution is a test rather than an instruction, and for most goals the first answer comes back higher than the money available. That is useful information, not a failure. It says the target and the deadline you typed are not compatible with the cash, and one of them has to give.

Four things can move, and they cost different amounts. Pushing the date back is usually the cheapest and the least popular. Cutting the target is the one people skip, though it often changes more than the headline: a deposit is not a single number, and the size of it changes what you have to borrow and repay afterwards, so a smaller target and a smaller mortgage can be the same decision. Raising the contribution is the obvious lever and the one with the real consequences elsewhere. Moving the money somewhere it earns more is the lever people reach for first and it is frequently the weakest.

You can find out which in about ten seconds. Set the growth rate to zero and read the monthly figure again. For a goal two or three years out it will barely budge, because almost the entire answer is what you put in: chase a better savings rate by all means, but the plan does not depend on it. For a goal ten or more years out the figure moves a lot, which means the assumed return is carrying part of the plan, and a plan carried by an assumption deserves to be run again at a lower one.

The bigger limitation is that this calculator assumes the goal is on its own, and goals never are. The deposit is competing with the wedding, the car that will not last another MOT, the emergency buffer and whatever the school year adds. Each of those has its own required monthly figure, and adding them up is the moment most plans fall over, because the same surplus was quietly promised to all of them. Working out the monthly figure for each goal separately and totalling them is the exercise worth doing before you commit to any single one.

Then the real question is what happens to the money that is left each month, and in what order. A rule that fills the emergency buffer to a floor first, then sends the surplus to the goal, survives a bad month far better than a fixed standing order that empties the current account. That is what CrestCast calls surplus cash routing, and it sits inside a forecast of the whole household, so you can see the goal, the bills and the debts drawing on the same months instead of one pot at a time.