Cohabitation Savings: How Much Moving In Together Saves

Two really can live cheaper than one

The saying is almost right. Moving in together doesn't halve your costs, but it doesn't double them either. Rent is the big lever: one shared home replaces two solo ones, so you pay for the larger place instead of both. Utilities, subscriptions, and household basics collapse toward a single set. Groceries scale the least — you still feed two people — but even there, bulk buying and less waste help.

Demographers have a name for this: the equivalence scale. The widely-used OECD square-root scale says a two-person household needs about √2 ≈ 1.41× a single person's budget — not 2×. Two solo renters spending $2,400 and $2,160 a month can drop to roughly $2,660 combined, freeing nearly $1,900 every month. Invested at 7% for a decade, that's a six-figure head start.

How the math works

1. Rent is counted once — the larger of your two solo rents (you move into one shared home).
2. Living costs (utilities, groceries, subscriptions, household goods) combine to (yours + theirs) × a sharing factor, default 70% — partial economies of scale.
3. Monthly savings = (both solo totals) − the combined household. Multiply by 12 for the annual figure.
4. Invested value projects those monthly savings forward 10 years at a 7% return, as an ordinary monthly annuity.

Source: the OECD equivalence (square-root) scale and the BLS Consumer Expenditure Survey on shared-household spending.

The simplification: the 70% living-cost factor is an average — your real number depends on how much you each already spent on shareable things. The rent model assumes you take the larger existing place; in practice a couple often rents a new two-bedroom that costs somewhere between the larger solo rent and the sum. Adjust the factor and rents to match your plan.

Math runs locally. Inputs never leave your browser. Source on github.

Where this calculation doesn't apply

• You upgrade when you move in. If a couple uses the savings to rent a nicer, pricier place, the cash saving shrinks — you're buying more home, not banking the difference. The model shows the saving, not what you do with it.
• One of you already owns. Moving into an owned home changes the math entirely — there's no second rent to drop, and mortgage, equity, and tax effects take over. Rent vs Buy fits that case.
• Long-distance or maintained second homes. If you keep two places for work, the rent-counted-once assumption breaks and savings are far smaller.
• Moving in is a relationship decision, not a budget one. The savings are real, but they're a side effect. Don't let a spreadsheet set the timeline of a relationship.

What to actually do

1. Enter each partner's current solo rent and other living costs — use your last couple of months of real spending.
2. Set the shared rent to what you'd actually pay together, not just the larger solo rent, if you plan to get a bigger place.
3. Decide up front what happens to the savings — banking or investing it is what turns "we save money" into actual wealth.
4. Run the invested-10-years figure to see the long-run stakes, then route it somewhere automatic before lifestyle creep absorbs it.