Why Do Retirement Calculators Give Me Different Numbers?

You run your numbers through two retirement calculators and get two different answers. One says you can stop working at 62. The other says 67 or later. You start wondering if one of them is broken.

Neither one is broken. They disagree because they make different guesses about inflation, how fast your money will grow, what percentage you can safely pull out each year, and when your Social Security or CPP/OAS checks will arrive. The arithmetic is the same in every tool. The assumptions wrapped around that arithmetic are not.

The Math Is Simple. The Guesses Are Not.

Every retirement calculator does the same core loop: take your savings, add investment growth, subtract what you spend, repeat each year until your benefits start, then add benefits and keep going. That loop is basic addition and subtraction stretched over 20 or 30 years.

But to run that loop, someone had to pick four numbers:

• Expected return. Will your portfolio grow 5%, 7%, or 9% a year before taxes? Nobody knows. A calculator has to guess.
• Inflation. Will a dollar cost 2% more each year, or 3.5%? That guess changes how far your savings stretch.
• Withdrawal rate. How much of your savings can you pull out without running out? Some tools use 4%. Others use 3% or 5%.
• Benefit start age. Do they assume you claim Social Security at 62, at 67, or not at all? Same question for CPP (as early as 60) and OAS (as early as 65, or deferred to 70).

Change any one of those four guesses and the answer shifts by several years. None of the guesses is wrong. But they are guesses, and they are doing most of the work.

Three Assumptions That Move Your Retirement Date the Most

Inflation: the quiet thief

If a calculator assumes 2% inflation, your $4,000-a-month spending goal stays at $4,000 for 30 years. If it assumes 3.5%, that same goal grows to about $11,200 a month by year 30. (Step by step: $4,000 × 1.035^30 ≈ $11,227.) That single guess can shift the answer by many years.

Withdrawal rate: 4% vs. 5%

The 4% rule says you can pull 4% of your nest egg in year one and adjust for inflation after that. Our 4% rule guide walks through where that number comes from and when it breaks. If a tool uses 5% instead, it thinks you need a smaller pile of savings to cover the same spending, so it says you can retire sooner. A one-point difference sounds tiny. On a $500,000 balance, it is the difference between $20,000 and $25,000 of annual income, which is a lot when your rent has not changed.

Benefit start age: 62 or 67

Social Security pays a smaller check if you claim at 62 versus your full retirement age. For people born in 1960 or later, full retirement age is 67. Claiming at 62 means a permanent cut of roughly 30%. The gap between a $2,000 check and a $2,800 check, stretched over 25 years of retirement, changes whether a calculator says "you are covered" or "you are short." Same story north of the border: CPP can start at 60 with a reduction, and OAS can be deferred to 70 for a larger monthly amount. See when to take Social Security or when to take CPP and OAS for the full rules.

Same Person, Three Calculators, Three Answers

Say Maria is 60, has $500,000 saved, spends $40,000 a year, and has no pension. Here is what three calculators might output depending only on their built-in guesses.

Assumption	Calculator A	Calculator B	Calculator C
Expected return	7%	4%	7%
Inflation	2%	2%	3.5%
Withdrawal rate	4%	4%	5%
Social Security start	67	67	62
SS monthly check (assumed)	$2,400	$2,400	$1,680
Gap during bridge years (no benefits yet)	$40,000/yr (60–66)	$40,000/yr (60–66)	$40,000/yr (60–61)
Ongoing gap once benefits begin	$11,200/yr	$11,200/yr	$19,840/yr
Calculator verdict	"Retire at 60, on track"	"Retire at 63, tight"	"Retire at 64, tight"

Read the first three rows. The math is identical once you plug the numbers in. Maria did not change anything. The tools just started from different starting lines. Calculator B is the cautious one because it assumes slower growth. Calculator C assumes Maria claims early and gets a smaller check, so her savings gap is wider.

None of those three is a scam. But none of them is a crystal ball either.

How to Spot a Reasonable Calculator (or Tool)

Before you trust any number on a screen, ask yourself three things:

1. Does it show its assumptions? A good tool prints "I used 7% return and 2% inflation" right next to the answer. If you cannot see the guesses, you cannot judge the result.
2. Does it count your benefits? Some free calculators ignore Social Security, CPP, OAS, or a workplace pension. They act as if you have only your 401(k) or RRSP. That makes every answer look worse than it is.
3. Does it account for the years before benefits start? If you stop working at 60 but Social Security does not pay until 67, you have seven years with no benefit check. A calculator that skips those years tells you to retire sooner than the math supports. See retire early if benefits start later for how those gap years change the picture.

When Can I Retire works out the age you can retire and how ready you are today, shows every step of the math, counts Social Security or CPP/OAS, pensions, taxes, inflation and the years before benefits start. You upload a statement or type your numbers, and you get a free check with the assumptions spelled out in plain text so you can see exactly what drove the answer.

What the Official Rules Actually Say

You do not have to guess the benefit part. The rules are fixed:

• Social Security: Earliest claim age is 62. Full retirement age is 67 for anyone born in 1960 or later. You can delay to 70 for a bigger check. The exact dollar amount depends on your earnings record and the cost-of-living adjustment, which changes every year. See ssa.gov for your personal estimate.
• CPP: You can start as early as 60, with a monthly cut of 0.6% for every month before 65. Delaying past 65 earns a 0.7% monthly boost up to age 70. See canada.ca for current amounts.
• OAS: You can start at 65, or defer up to 70 for a 36% larger monthly payment. Income above a threshold (set each year by the government) can reduce or eliminate your OAS through the recovery tax. See canada.ca for the current threshold.

Because the dollar figures shift yearly, any calculator that stamps a hard maximum or limit into its code is already slightly outdated. The ones that pull from official sources or ask you to type in your own estimate are safer.

What to Do With the Spread

If two tools give you answers five years apart, that spread is useful information. It means your retirement date is sensitive to something: a slow decade in the market, an inflation spike, or a decision about when to claim benefits.

A few steps help:

• Run the conservative version. Plug in a lower return (4-5%) and a higher inflation (3-3.5%). If you are still roughly on track, the optimistic number was probably fine. If you fall behind, you know which assumption is doing the damage.
• Decide your benefit start age before you run the numbers. If you plan to take Social Security at 62 because you want to stop working sooner, tell the calculator 62. If you plan to wait until 67, tell it 67. Do not let the tool pick for you.
• Update your inputs every year. A raise, a market drop, a new child's college cost, a mortgage payoff. Small changes compound. How much do I need to retire? gives a starting framework you can revisit each year without overhauling your plan.

You do not need the one true number. You need a range you can act on, and you need to know which knob to turn if that range shifts. A calculator that shows its assumptions and lets you change them is doing its job. One that hides them is giving you a fortune, not a forecast.