Why a spreadsheet at all?
Because we needed something we could change.
Online calculators were useful for isolated checks, but our real question was bigger: when could the two of us stop working and fund the life we wanted? To answer that, pensions, ISAs, mortgage, tax, State Pensions, Karen’s teacher benefits, retirement timing and spending all had to appear in the same story.
The calculators got us only so far
My first spreadsheet was brutally simple: pension money, future contributions, an assumed return, then an annuity at 67. It produced an answer. It just was not an answer to our actual life. It was also a bit disappointing.
The online tools were good at answering the question they were built for. Our problem was that we kept asking follow-up questions. What if we retired earlier? What if we cleared the mortgage first? What if we spent more in our sixties and less later? What if one assumption was wrong?
What the spreadsheets let us do
They made the assumptions visible. More importantly, they let us change one and run the whole thing again.
Some of the variables we tested were our retirement date, pension contributions, investment returns, mortgage timing, monthly saving, ISA growth, State Pension dates, teacher pension income, annuity income, tax thresholds and how much we wanted to spend at different ages.
Change one assumption. Run it again. Change another. Run it again. We built a spreadsheet, changed it, changed it again, made choices, found mistakes, fixed them and tried again. That was the point.
The rule that survived all the tinkering
Every apparent improvement had to survive the same questions: is it real, is it meaningful, and does it still work if we change the assumptions?
Sometimes a brilliant-looking result was a genuine improvement. Sometimes we had double-counted money or broken a formula. Nothing punctures financial genius quite like finding the same £20,000 twice.
What they actually gave us
The spreadsheets did not predict the future and they certainly did not create money. They gave us a way to trace our assumptions, test alternatives and understand what was actually moving the answer.
That mattered more than the final total. We could see which decisions made a real difference, which clever-looking optimisations barely moved anything, and which choices were about us rather than maths.
Thirty-seven spreadsheets later, we finally had a plan we could explain. More importantly, we had one we knew how to challenge.
We tried turning the spreadsheet into an app.
It became a much bigger problem than “make the spreadsheet easier to use”. After repeated build-test-fix cycles, we stopped.
Read what happened →It reflects our ages, pensions, mortgage, tax position, priorities and assumptions. The useful bit is the questions we asked—not copying our answer.
Official information we checked
Rules change. These links are for checking the underlying rules rather than treating our experience as guidance.