The Smart Spending Rule

How to get the most happiness out of every dollar.

4 min readQuick check inside

The big idea

You get the most out of your budget when the last dollar you spend on each thing gives you the same amount of satisfaction.

Everyone has a budget constraint, meaning a fixed amount of money and prices they can’t control. Say Taylor Swift has $500 for coffee ($5) and concert outfits ($50). She can buy 100 coffees, 10 outfits, or any mix in between. Every one of those mixes spends the full budget, so the real question is which mix makes her happiest.

The answer is the optimal consumption rule: keep shifting money toward whatever gives you more satisfaction per dollar until both are equal. If coffee gives 10 units of joy for $4 (2.5 per dollar) and smoothies give 15 for $6 (also 2.5 per dollar), you’re balanced. If one gives more joy per dollar, buy more of it.

When a price changes, your best mix changes too. If pasta gets pricier, tacos start looking better and you switch. That’s the substitution effect. Higher prices also make your money feel smaller. That’s the income effect, and it’s why price hikes on big things hurt so much.

Spot it in the wild

GroceriesWhen beef jumps in price, chicken suddenly gives more joy per dollar, so people switch.
A raiseMore income shifts your whole budget line out, so you can afford more of everything.
MenusRestaurants design combos and “decoy” sizes to change how you weigh value per dollar.

Quick check

Tap an answer.

Snacks give you 6 utils per $2 and sodas give you 4 utils per $2. What should you do?

Snacks give 3 utils per dollar against 2 for sodas. Shift spending toward snacks until the two even out.

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