A junior SIPP calculator projects a small regular contribution across five decades and produces a large number. The arithmetic is right and the number is not a forecast. Over that length of time the assumed growth rate does more work than the contributions do, so the useful skill is reading what the calculator has assumed rather than admiring what it has produced.
Why the growth assumption dominates
Compounding is exponential in time, so a projection running fifty years is extraordinarily sensitive to the rate applied. Moving an assumed annual return by a single percentage point can change the final figure by a large multiple, not a small margin, because the difference compounds every year for half a century. A calculator that lets you change the rate is more honest than one that hides it, and the first thing to do with any of them is to run the same contribution at a materially lower rate and see what survives.
What is usually left out
Three things commonly go missing. Charges are the first: a platform fee and a fund charge taken every year reduce the compounding rate directly, and a projection gross of charges is not the money anyone receives. Inflation is the second: a large number in fifty years is not a large number in today's terms, and a projection in real terms is far more useful than one in nominal pounds. The third is contribution behaviour, because almost nobody pays the same amount for fifty years without a gap.
What the calculator is genuinely good for
It is a poor forecaster and an excellent comparator. Run the same assumptions with a start at birth and a start at age ten and the gap between them is the value of the years, which is the one thing about a junior pension that is not speculative. Run a small monthly amount against an occasional lump sum and you can see which suits a family's cash flow. Used to compare decisions against each other rather than to predict a balance, it answers the question people actually have.
The limits of any projection
No calculator knows future returns, future charges, future inflation or future rules, and a pension started for a child today will live through many changes to all four. This site publishes the mechanics rather than a promise, does not advise on whether to open one, and does not recommend a provider. Where a family wants advice on its own circumstances, the enquiry goes to a firm authorised to give it.
Questions people ask about junior SIPP calculator
Are junior SIPP projections reliable?
They are reliable arithmetic on assumptions nobody can verify. Over fifty years the assumed growth rate matters more than the contributions, so treat the output as a comparison between choices rather than as a prediction of a balance, and always rerun it at a lower rate.
Should the projection be shown after inflation?
It is far more useful that way. A large nominal figure fifty years out says little about what it will buy. A projection in today's money, after an inflation assumption, is the version worth acting on, and a calculator offering both is telling you something a nominal-only one is hiding.
Do charges make much difference over that long?
A great deal, because they reduce the compounding rate itself every year. A projection calculated gross of platform and fund charges will overstate the outcome substantially over five decades, so check whether the calculator has taken them off before comparing providers on the result.
Does starting at birth really matter?
It is the clearest thing a calculator can show. Running identical assumptions from birth and from age ten isolates the value of ten extra years of compounding, and that gap is the strongest argument for the product, because time is the only input that cannot be added later.