A spreadsheet is a perfectly good retirement planner right up to the point UK tax rules enter the model. Here is exactly where the line falls, and how to tell which side of it your plan sits on.
Most people planning their own retirement start in a spreadsheet, and for good reason. It is free, you can see every assumption, nothing is hidden behind someone else's product decisions, and you are not handing your finances to a website. If you want to know roughly whether a pot lasts, a spreadsheet will tell you in twenty minutes.
We should be straight about our position: this site is published by the company behind FIRElogic, a paid retirement planner. So rather than tell you spreadsheets are inadequate, this page sets out precisely which parts of a UK retirement plan a spreadsheet handles well, and which parts are genuinely hard to do in one. You can then judge which side of that line your own plan falls on.
The core of a drawdown projection is not complicated. A pot grows at some rate, you take an income from it, inflation erodes what that income buys, and you want to know whether the balance survives to a plausible age. That is one row per year and about four formulas. A spreadsheet is an excellent tool for it, and anyone telling you otherwise is selling something.
Specifically, a spreadsheet is entirely adequate for:
One pension, one growth rate, one income figure, inflation-adjusted. Genuinely a five-minute job, and the arithmetic is transparent enough that you will trust the answer — which matters more than people admit.
Change the growth rate from 5% to 3% and watch what happens. Spreadsheets are very good at this, and playing with the inputs yourself builds an intuition for which assumptions actually drive the outcome. Most people discover the growth rate matters less than the retirement date.
Running the model backwards — what do I need at 60 to draw £30,000 — is straightforward, and a spreadsheet handles it as well as anything.
If your plan is one pension, one person, and a flat income, stop reading. A spreadsheet is the right tool and you do not need software. The problems below only start once tax enters the model.
The difficulty is not the projection. It is that the amount you can actually spend depends on tax, and UK pension tax has several interacting rules that resist a simple formula. Each is manageable alone; together they compound.
You do not want to know what £40,000 of withdrawals leaves you after tax. You want to know what to withdraw to end up with £30,000 to spend. That is the inverse problem, and because tax is a piecewise function of income there is no clean algebraic answer — you have to search for it. In a spreadsheet that means either iterating by hand until the number settles, or building a solver. Multiply by forty years and it becomes the dominant cost of maintaining the model.
Take money from a defined contribution pension and, broadly, 25% arrives free of income tax and 75% is taxable. That is easy enough to encode. What is harder is that it changes the optimal order in which you draw from your pots, because the tax-free element interacts with your personal allowance differently depending on how much other income you have that year.
At State Pension age a fixed, triple-locked, taxable income appears and occupies most of your personal allowance. Every pound you draw from the pension after that point is taxed from the first penny, so your net income per pound withdrawn falls sharply at a single point in the plan. A spreadsheet can model this, but the year it happens is a discontinuity, and it moves if the rules or your age change.
With a pension, an ISA and taxable savings, the order you spend them in changes your lifetime tax bill — sometimes by a great deal, because ISA withdrawals are tax-free and do not consume allowances, so they can be used to top up income without pushing you into a higher band. Finding the best order is an optimisation problem across the whole plan, not a single formula, and it is the part almost no spreadsheet attempts.
Allowances and bands are periodically frozen, raised, or withdrawn above certain incomes. A spreadsheet encodes whatever was true when you built it. There is no mechanism to tell you a threshold changed, which means the model quietly becomes wrong rather than visibly breaking.
Two people are not one person with a bigger pot. Each has a personal allowance and their own set of bands, so a household drawing a given income between two people keeps more of it than one person drawing the same amount alone. Getting that right means modelling both people separately and then coordinating withdrawals across them — roughly doubling the model and adding the question of who draws what, in which order.
There is also a factor people miss entirely: a couple receives two State Pensions. In most household plans that second entitlement accounts for more of the difference than tax sequencing does, and it depends on both partners having a full National Insurance record. Individual forecasts are available on GOV.UK, and they are what a household projection ultimately rests on.
A reasonable test: write down how many of these apply to you.
One pension, one person, a flat income target, and you are content with an answer that is roughly right on tax. You will not get a better understanding of your own plan than by building it yourself.
You are planning as a couple; you hold more than one wrapper and want the withdrawal order optimised rather than guessed; you are retiring well before State Pension age so there is a long bridge to fund; or you want the tax calculation maintained for you rather than being the thing you have to keep an eye on.
The honest summary is that a spreadsheet models the investments well and the tax badly, and which of those dominates depends entirely on how complicated your affairs are. Plenty of people genuinely do not need more than a spreadsheet.
There are several circulating on UK personal finance forums, and the better ones handle growth, inflation and a flat withdrawal competently. What almost none of them do properly is solve gross-to-net income under UK tax, or optimise which pot to draw from first. Treat a downloaded spreadsheet as a projection of your investments rather than a statement of what you can spend, and check whose tax year the assumptions were written for.
Yes, with effort. Because tax is piecewise, you need to search for the withdrawal that produces your target net income rather than compute it directly — in practice Goal Seek, or an iterative column. It works for one year. Doing it for every year of a forty-year plan, for two people, and keeping it correct as thresholds change, is where the maintenance burden becomes real.
Two things, in our experience of reviewing them. It applies a single average tax rate rather than modelling bands and allowances, which flatters early-retirement plans where income is low. And it ignores drawdown order, so it silently assumes a withdrawal strategy that is rarely the cheapest available.
No, and we would not recommend it. A spreadsheet you built yourself is the best way to understand which assumptions your plan is actually sensitive to. Its most useful role afterwards is as a sanity check: if a tool disagrees with your own model by a wide margin, one of you is wrong, and finding out which is time well spent.