Most people have a vague plan for retirement. It usually goes something like this: the state pension will cover the basics, and there will be a bit saved on the side to make life comfortable. It is a reassuring plan. It is also worth checking, because the maths behind public pensions has been shifting quietly for the last thirty years.
This is arithmetic, not politics
A pay-as-you-go pension works in a simple way. The people working today pay contributions, and that money goes straight to the people who are retired today. Nothing is invested in your name and set aside for later. It is a transfer from one generation to the next.
That works well when there are plenty of workers for every retiree. In France during the 1960s there were roughly four people working for each pensioner. Today the ratio is closer to 1.7, and the projections keep sliding because people live longer and fewer children are being born. You do not need a strong opinion about any government to see the issue. Fewer workers paying for more retirees, for more years each, means the promise gets harder to keep at the same level.
Orders of magnitude: the ratio depends on how "worker" and "retiree" are defined, and serious sources do not agree to the decimal. What all of them show is the slope. A pay-as-you-go pension is funded by this ratio and by nothing else.
Governments respond in the only three ways available to them. They raise the retirement age, they raise contributions, or they let the replacement rate drift down so that pensions cover less of your old salary than they used to. We have already seen all three. There is no reason to expect that to stop.
What this means for you
None of this says the state pension will vanish. It probably will not. The more realistic worry is that it covers a smaller slice of your needs than you are assuming, and that you find this out at the worst possible moment, a few years before you stop working, when you have very little time left to fix it.
The honest move is to find out now, while you still have decades of compounding ahead of you. That means putting two numbers next to each other. The first is what you will actually spend per month once you retire. The second is what your pension is realistically projected to pay. The difference is your gap, and the gap is what your own savings have to fill.
For a lot of people the gap is bigger than they expect, because they quietly assume their pension will replace most of their salary. For higher earners especially, the replacement rate can be surprisingly low. The system is more generous, in relative terms, to people who earned less.
How to size the gap
Start with your spending, not your income. Look at what actually leaves your account in a normal month, then strip out the things that disappear in retirement, like your commute or a mortgage you will have paid off. Add the things that grow, like health costs. That gives you a target monthly figure in today's money.
Then get a rough projection of your pension. In France you can pull an estimate from your retirement account online. It will not be exact, but it does not need to be. You are looking for the shape of the problem, not a figure to the nearest euro.
Subtract one from the other. If your target is 3,000 per month and your pension is projected at 1,800, your savings need to produce 1,200 per month, adjusted for inflation, for as long as you live. The classic rule of thumb says you need roughly 25 times the annual gap invested to draw it down safely. A gap of 1,200 per month is 14,400 per year, so around 360,000 of invested capital. That is a real number you can plan around, and it is far less frightening when you are looking at it at 35 than at 60.
Why a tool helps here
The reason most people avoid this exercise is not laziness. It is that doing it by hand is fiddly and a little depressing, and our brains are very good at deciding to think about it later. Software does not get tired or discouraged. It just shows you the gap, updates it when your situation changes, and lets you test what happens if you save a bit more, retire a year later, or earn a slightly lower return than you hoped.
That is the whole point of running the projection early. Not to scare yourself, but to turn a vague worry into a specific number, and to give yourself the years you need to do something about it. The people who reach retirement comfortably are rarely the ones who earned the most. They are the ones who looked at the gap early and kept filling it.