How to Calculate Your Retirement Number | SIP Retirement Calculator Guide

Retirement Planning · SIP Method™

You Have the Life.
Now Get the Number.

A guide to calculating your retirement number — and closing the gap

Most people spend decades working toward retirement without ever knowing the actual number they need to get there. Here's how to find yours — and what to do once you see it.

📖 8 min read 🧮 Includes worked example 🛠 Free calculator included

Nobody sat you down and showed you this. That's not a character flaw — it's just how it went.

If you grew up in a household where money was either scarce or simply not discussed, the mechanics of long-term wealth can feel like a foreign language. You work. You save what you can. You tell yourself you'll figure out the retirement piece later. Later arrives and you still don't have a number.

The people who seem to have it together financially aren't smarter than you. Most of them just got shown something early — a parent who explained compound interest, an employer who walked them through a 401(k) match, an advisor who handed them a projection. They got the information. You didn't. That's the gap we're here to close.

"The number isn't something to be afraid of. It's the thing that makes every other decision clearer."

The SIP Retirement Calculator exists for exactly this moment. Not to overwhelm you with projections and jargon. To show you one honest number — the nest egg you need — and walk you through where you stand against it right now.


Step 1 of 3 Start with the life, not the math

Before the calculator asks for a single dollar figure, it asks you something different: what does a good life actually look like for you in retirement?

You pick up to three things from a list — traveling freely, time with family, pursuing projects you care about, staying active, a home you love, supporting causes that matter, not lying awake worrying about money, leaving something behind.

This isn't decorative. It's the most important input in the whole tool.

Here's what happens when people skip this step and jump straight to the numbers: they build a plan toward a target that doesn't match the life they actually want. They hit the number — or don't — and neither outcome feels right because the destination was wrong from the start.

Your retirement isn't a generic financial event. It's a specific life. The plan should be built around that life — not around someone else's idea of what retirement looks like.

Take thirty seconds with this question. Be honest. Your three picks become the lens through which every other number in the calculator makes sense.


Step 2 of 3 Three numbers. That's it.

The calculator asks for exactly three inputs to generate your retirement target:

  • 1
    Monthly spending in retirement — in today's dollars. What would it actually cost you to live the life you just described? Housing, food, healthcare, travel, the things that matter. Don't guess low.
  • 2
    Years until you retire — your honest best estimate. You can always come back and adjust.
  • 3
    Years in retirement — this one surprises people, but it's critical. You choose 20, 30, 40, or 50 years.

That third number drives your withdrawal rate — the percentage of your nest egg you can safely spend each year without running out of money:

5%
20-year retirement
4%
30-year retirement
3.5%
40-year retirement

The logic: a longer retirement means your money needs to last longer, so you take less each year to keep it intact. Plan for 30 years at $8,000/month and you need a very different number than planning for 20 years at the same spending. The calculator adjusts automatically based on what you select.

The calculator also adjusts for 3% annual inflation automatically. Enter what things cost today. The math handles what they'll cost in 20 years.

When you hit Calculate, you'll see your number. A lump sum — the nest egg you need accumulated by the day you stop working to fund your monthly lifestyle, at your chosen withdrawal rate, for as long as you said you'd live. It may be bigger than you expected. That's okay. A coordinate you can see is always better than a fog you can't navigate.


Step 3 of 3 Enter what you have — all of it

This is where the calculator does something most tools never bother with.

You enter your assets — not just your retirement accounts, everything. Home equity. Rental property. Investment accounts. Business value. Savings. And for each one, you flip a simple toggle: Growing or Idle.

Growing means the asset can generate investment returns. Your 401(k), your brokerage accounts, your rental equity — these are working toward your number. Your primary home? Usually Idle. Cash sitting in a savings account at 0.5%? Idle. This distinction — what we call Growing Capital — is the subset of your wealth that's actually in motion toward your goal.

Then you enter your liabilities — mortgage, car loans, credit cards, student debt. The calculator nets everything out to your true position.

Then it asks about future one-time expenses before retirement: college costs, a parent's care, a home renovation. These compete with every dollar you're trying to accumulate, so they belong in the picture.

And finally — the section that changes the calculation for a lot of people — monthly income in retirement. Social Security. A pension. Rental income. Part-time consulting. Any guaranteed or expected income that'll come in each month.

Every $1,000/month of retirement income is worth $300,000 in savings you no longer need to accumulate — at a 4% withdrawal rate.

That math is called the income corpus offset, and it's real. A $2,000/month pension removes $600,000 from your gap before you've done anything else. Many people discover their actual gap is a fraction of what they feared — once income is accounted for properly.


What this looks like in practice

💼

The Mid-Career Professional Juggling Everything

42 years old · 18 years to retirement · wants to retire at 60

She maxes her 401(k) every year and still feels behind. She has never calculated her number because the idea felt overwhelming. She opens the calculator and works through it.

Monthly spending goal
$12,000/mo
Years in retirement chosen
35 years
Withdrawal rate applied
3.5%
Retirement target
$4.9M

She enters her assets: $380,000 in her 401(k) (Growing), $120,000 in a brokerage account (Growing), $280,000 in home equity (Idle), $40,000 in savings (Idle). Net worth after her mortgage: $820,000. Growing Capital: $500,000.

She adds future needs: $120,000 for her daughter's college in 6 years. Total need rises to $5.02M.

Then she enters her expected Social Security: $2,400/month. The income corpus offset at 3.5%: $823,000 removed from her target. Real gap: $4.2M minus $820,000 net worth = $3.38M to close.

She enters $3,500/month in savings, and selects 6% annual growth. The projection: her existing net worth grows to $2.64M over 18 years. Her monthly savings compound to $1.26M. Total projected: $3.9M. She is ahead of her income-adjusted target.

✅ She wasn't behind. She just didn't have a picture. The calculator gave her one in under 10 minutes.

The number doesn't judge you. It navigates you.

Whatever comes up when you run your own numbers — whether it's relief or a reality check — the point is that you're no longer guessing. You have a coordinate. You know what the gap actually is, not the version of it you've been carrying around in your head.

A gap you can see is a gap you can close. And closing it is not just about saving harder — it's about making sure the money you already have is arranged to work for you. Growing Capital instead of idle assets. Income that offsets what you'd otherwise need to accumulate. A monthly savings number that actually moves the needle.

That's what the calculator shows you. And it's where a real financial plan starts.

Calculate My Retirement Number →

This calculator is for educational purposes only and does not constitute investment advice. Results may be inaccurate and are intended as guidance only. Sustainable Investment Partners LLC is a Registered Investment Advisor based in Lexington, MA.