You want to retire at 50. You have £260,000 across a SIPP and a S&S ISA and you save £2,200 a month. Personal Capital will run a Monte Carlo simulation against your US-style returns assumption. Delphina will give you a date based on UK rules and your real spending. Here is which is more useful for FIRE.
Personal Capital's Retirement Planner can be aimed at an early retirement age. You set your target age, your expected spending in retirement, and your current portfolio, and the Monte Carlo simulation tells you the probability of not running out of money by age 95. The simulation uses historical US market returns and US inflation paths.
For a US household targeting FIRE, the tool is honest about sequence-of-returns risk and the danger of withdrawing too much in a downturn. It is less useful when applied to a UK household because the asset return assumptions, the inflation path, and the absence of any State Pension modelling all reduce the reliability of the output.
Delphina calculates your FIRE number as 25 times your real annual spending, then projects forward using your actual savings rate and expected returns on a UK asset mix. The State Pension forecast is built into the projection as income from your State Pension start date, which reduces the portfolio you actually need to accumulate by £12,548 a year (the current full new State Pension).
For a UK saver with £260,000 invested and £2,200 a month going in, Delphina gives you a date: "based on your current trajectory and a 5% real return assumption, you reach 25x your spending at age 49 and 8 months." If you want to retire at 45 instead, the tool shows you the additional monthly contribution that brings the date forward, and the trade-off if you cannot find that money.
Personal Capital answers "what is my probability of success at age X?". Delphina answers "at my current pace, when do I hit the number?". The first is useful when you have built the portfolio and want to test drawdown strategies. The second is useful when you are still accumulating and want to know whether the goal is realistic and what changes it.
For most people targeting FIRE, the second question is the one that matters. Knowing you need to save £600 a month more to retire at 50 versus 55 is more useful than knowing your Monte Carlo probability shifts from 76% to 89%.
Add up your last 12 months of actual spending from your bank statements. Multiply by 25. That is your FIRE number today, no app needed. Connect Delphina this week to find the date you reach it at your current pace.