The 90/10 Rule: How to Invest Boringly and Still Scratch the Itch

Boring investing works. But let’s be honest: it is boring. If you have any interest in markets, the urge to buy that one stock, that one coin, the thing your friend won’t stop talking about, is strong. Fight it completely and you might rage-quit your whole plan. The 90/10 rule is how you scratch that itch without torching your future.

What the 90/10 rule is

Simple: put 90% of your investing money in low-cost index funds, and keep 10% for “fun money” bets.

Quick note: this is different from Warren Buffett’s famous 90/10 (90% in an S&P 500 fund, 10% in short-term bonds). This version is for people who want to mostly invest the smart, boring way but still enjoy taking a few swings.

The 90%: the foundation

The big chunk goes into low-cost index funds, the globally diversified, self-cleansing, low-fee vehicles that quietly compound for decades. This is the part that actually builds your wealth. It is diversified, cheap to own, and has a long track record of steady long-term growth. Most of your money should be doing this unglamorous work.

The 10%: fun money

The other 10% is yours to play with. Individual stocks, crypto, a startup, whatever you find exciting. The rule for this bucket is one line: it is money you can afford to lose. Treat it like an entertainment budget, not a retirement plan. If it works out, fantastic. If it goes to zero, your real portfolio never even noticed.

Why it works

The 90/10 rule works because it manages the most dangerous part of investing, which is your own emotions.

The biggest reason people wreck their finances isn’t picking the wrong index fund. It is letting FOMO take over, dumping their savings into a hot bet at the top, then panicking and selling when it drops. The 90/10 rule gives that urge a safe place to live. You get to take a swing, feel the excitement, and learn the lessons, all inside a 10% sandbox that can’t sink the ship.

It also keeps you calm. When 90% of your money sits safely in diversified index funds, a bad week in your fun bucket is a shrug, not a crisis. That peace of mind is what keeps you invested for the long haul, which is where the real money gets made.

A quick story

A few years ago a friend put almost everything she had into one hot tech stock. It tripled. She felt unstoppable. Then it gave it all back and then some, and she swore off investing entirely, right before the market went on a long run she missed out on.

Now picture the 90/10 version. Same hot pick, same crash, but it was only 10% of her money. The other 90% kept compounding in index funds the whole time. She still got to take the swing, learned the lesson cheaply, and never stopped investing. That is the entire point.

How to use it

1. Pick a low-cost all-in-one index ETF for your 90%, and buy it inside the right account.
2. Automate the contributions so the 90% happens without you thinking about it.
3. Keep your 10% fun money clearly separate, and never top it up from the 90% when you get excited. That boundary is the whole discipline.

One honest caveat: for some people the right amount of fun money is 0%. If you know you can’t stop at 10%, or a loss would genuinely hurt you, there is no shame in putting everything in index funds and skipping the casino entirely. The 90/10 rule is a release valve, not a requirement.

This is a general framework, not personalized investment advice. How you split your own money depends on your situation and your tolerance for risk.

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Want help building your boring, effective 90%, and the discipline to keep the 10% in its lane? That’s what the 4-week coaching program is for. Or model your long-term growth first with the Compound Interest Calculator.