US Social Security + UK State Pension: The Complete Calculator Guide

The Windfall Elimination Provision is gone. If you've been putting off modelling your US and UK pensions together because WEP made the maths a guessing game, that excuse disappeared in January 2025. What's left is simpler than most people expect: two entries in RetireFlexi, five minutes, and one combined number in whatever currency you actually think in.

This is the field-by-field version. Not the legal background — that's covered here if you want the Totalization Agreement and the old WEP mechanics explained properly. This post is just: which fields, in what order, and where people get it wrong.

What changed in January 2025

The Social Security Fairness Act repealed WEP and its cousin the Government Pension Offset. Before the repeal, drawing a foreign state pension — the UK State Pension included — could knock a few hundred dollars a month off your US Social Security. That reduction no longer applies. If you retired or started drawing benefits before the repeal, the Social Security Administration has been issuing retroactive back payments through 2025; if you haven't checked your account for one, it's worth five minutes on ssa.gov.

Practically, this means you enter your Social Security amount as-is. No haircut, no estimate of "what WEP probably takes off." Just the number from your statement.

Before you open the calculator

Get two numbers first, both from the source, not from memory.

• US Social Security: log into ssa.gov/myaccount and pull your estimated monthly benefit at the age you're planning to claim. The statement gives you figures for 62, full retirement age, and 70 — grab whichever matches your plan, or all three if you're not sure yet.
• UK State Pension: log into the Government Gateway at gov.uk/check-state-pension and get your forecast. This tells you your qualifying years out of 35 and your forecast weekly amount — which already accounts for any gaps in your National Insurance record.

Don't skip this and rely on the calculator's defaults. When you select a country in the state pension section, it auto-fills a population-average figure — useful as a placeholder, wrong for your actual retirement plan. The 2026/27 UK default is £12,547, the full new State Pension rate for someone with all 35 qualifying years. If you've got 28, your real number is smaller, and only your own gov.uk forecast tells you how much.

Adding your US Social Security

In Assets & Income, open the section labelled Government Pensions. (Yes — the help text and the FAQ on this site call it "State Pensions," the actual section header on screen says something else. Same thing. It trips people up for about four seconds.)

1. Click + Add
2. Set Country to United States — this loads the 2026 default ($24,852/year full rate, $17,400 if claimed at 62), which you're about to overwrite
3. Set Draw Age to whatever age you pulled your estimate for — 62, 67, or 70
4. Replace Annual Amount with your actual SSA figure, annualised (monthly × 12)
5. Set Amount Basis to Today's Money — your SSA statement is already in today's dollars, so let the calculator handle the inflation forward from here
6. Leave Growth Rate at the COLA default (around 2.8%) unless you have a reason to model something different

Adding your UK State Pension

Click + Add again for a second row. Same section, second entry.

1. Set Country to United Kingdom
2. Set Draw Age to your State Pension age — for most people currently planning retirement, that's 67. Unlike Social Security, there's no early-claim option here. You can defer past State Pension age for a higher weekly rate, but you can't pull it forward the way you can with US benefits at 62.
3. Replace Annual Amount with your gov.uk forecast figure, annualised (weekly × 52)
4. Set Amount Basis to Today's Money, same logic as above
5. Leave Growth Rate at the triple-lock default (around 3%)

Today's Money vs Future Money — which one to pick

Both your SSA statement and your gov.uk forecast are expressed in today's terms — what the benefit would be worth if you started drawing it right now, not what it'll actually look like as a number in 15 years after inflation. That's why Today's Money is the right basis for both entries. The calculator takes it from there: your figure gets inflated forward from your current age to your draw age using the general inflation rate, then escalated annually at whatever growth rate you set once payments start.

Get this backwards — enter your statement figure as Future Money — and the calculator treats it as the actual nominal payment starting now, which understates what you'll really receive by however many years of inflation separate today from your draw age. For someone 15 years out, that's not a rounding error.

Comparing claiming early against deferring

This is where it stops being two static numbers and starts being a decision. Both pensions reward waiting — Social Security roughly 8% per year past full retirement age up to 70, the UK State Pension around 5.8% per year past State Pension age — and the only honest way to compare "claim now" against "wait" is to run both and look at the difference.

Set your current plan as Scenario A. Then change the Draw Age on one or both pensions — say, US Social Security from 67 to 70 — and click Save Current as Scenario B. The two now sit side by side: Year 1 income, portfolio longevity, Monte Carlo success rate, all compared directly. If deferring drops your Year 1 income by $8,000 but adds four years of portfolio survival, that's a real trade-off you can actually see, not a hunch.

Reading the combined result

The Dashboard is where the two pensions stop being separate line items. It shows your guaranteed income floor — both state pensions added together, converted into your reporting currency — sitting underneath whatever you're drawing from investments. That floor is the number that doesn't move regardless of what markets do in any given year.

Alongside it: a withdrawal rate on the portfolio gap, and a Monte Carlo success probability built from 5,000 simulated market paths. The RAG dots (green, amber, red) give you the fast read; the actual percentage is what matters if you're deciding whether to adjust anything.

What the calculator still won't do for you

Two things, and they're worth knowing rather than discovering later.

First, tax. RetireFlexi applies the tax bands of a single country — whichever one you set as your Country of Tax Residence. It doesn't model the US-UK tax treaty, which determines which country actually taxes your Social Security or your State Pension depending on where you're resident when you draw it. That's genuinely complicated, particularly if you hold US citizenship, since the US taxes worldwide income regardless of where you live. The original guide covers the treaty mechanics; this calculator doesn't replace a cross-border tax adviser, and it isn't trying to.

Second, exchange rates are fixed for your session. Live rates convert your GBP pension into USD (or vice versa) the moment you load the calculator, but they don't shift year by year across a 20 or 30-year projection the way real currency markets will. If a meaningful share of your retirement income arrives in a currency you don't spend in day to day, that's real uncertainty sitting outside the model. Worth remembering before you treat any number to the dollar as settled.

A worked example

Someone with 30 qualifying UK years out of 35, and a US Social Security statement showing $2,200/month at full retirement age of 67.

No haircut on the US side — that's the actual difference the 2025 repeal makes to this specific example. Enter both rows in RetireFlexi with Draw Age 67 and Today's Money basis, and the Dashboard shows the combined guaranteed floor in a single reporting currency, sitting under whatever the investment portfolio contributes on top.

Common mistakes

• Leaving the country default in place. The auto-filled amount is a population average, not your entitlement. Always overwrite it with your own statement figure.
• Entering a UK Draw Age below State Pension age. There's no early-claim option in reality, so a Draw Age of 60 for the UK pension models a scenario that doesn't exist under current law.
• Mixing Today's Money and Future Money across the two rows. Pick one basis per entry deliberately — both statements are already in today's terms, so Today's Money is correct for both in almost every case.
• Forgetting the WEP reduction is gone. Old advice, old blog posts (including earlier versions of this site's own content), and old spreadsheets may still tell you to knock a few hundred dollars a month off. Don't. Enter the full SSA figure.