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Two singles living apart vs one couple sharing costs, and what that monthly "Synergy Surplus" compounds to.
All figures are in today's money: every rate on this page is real, i.e. above inflation. Why?
Individual (each, per month)
Couple (shared, per month)
Investment
| Metric | P10 | P50 | P90 |
|---|---|---|---|
| Single Sum Cost | – | €3,500 | – |
| Couple Cost/mo | – | €2,400 | – |
| Synergy Surplus/mo | – | €1,100 | – |
| Nest Egg (20 yrs) | €261,409 | €448,204 | €833,307 |
Monthly cost: two singles vs one couple
Two people living apart each pay rent, groceries and utilities on their own; a couple pays each bill once. The gap between the bars is the Synergy Surplus invested below.
Synergy surplus, invested
The monthly Synergy Surplus compounds at 5.0%/yr (median path); the shaded band is the modelled P10–P90 range under volatility σ = 15%.
- Nest egg P10–P90 · 80% of scenarios: The middle 80% of simulated futures: 1 run in 10 ends above this ribbon and 1 in 10 below it.
- Nest egg P25–P75 · middle half: The middle half of simulated futures: a quarter end above it, a quarter below.
- Nest egg median (P50): The median (P50): half of all simulated futures end above this line, half below. It coincides with the solid line.
- solid = median (P50) path: The solid line is the median (P50) path. Because returns compound, the arithmetic mean sits ABOVE the median: a few very good runs pull the average up.
The difference between the two bars on the left is the surplus that compounds into the nest egg on the right.
How the scenarios work: the shaded band models the invested pot as compounding at a lognormal return with median 5% and volatility σ = 15% (set in the header). Only the invested pot is randomised; the monthly synergy surplus stays fixed at your rent/grocery inputs. The band is the resulting P10–P90 / P25–P75 range; the solid line is the median (P50) path.
Two singles, combined monthly cost
singlesTotalMonthly = 2 × (1,200 + 400 + 150)
- indRent: monthly rent, each single
- indGroceries: monthly groceries, each single
- indUtilities: monthly utilities, each single
One couple, shared monthly cost
coupleMonthly = 1,600 + 600 + 200
- sharedRent: monthly rent, shared
- sharedGroceries: monthly groceries, shared
- sharedUtilities: monthly utilities, shared
Synergy surplus
- singlesTotalMonthly: see above
- coupleMonthly: see above
Nest egg (invested synergy surplus)
- r: monthly market rate = (1 + marketReturn/100)^(1/12) − 1
- m: month index, 1…horizonYears×12 (clamped to 1-40 years)
Assumptions behind every figure: how this site models the market →
Every model leaves things out. Here is what this one does not see:
- No breakup or separation risk. The model assumes the shared arrangement holds for the whole horizon; a split resets both partners to single costs and forces the nest egg to be divided, so the compounding curve overstates what any one person actually ends up keeping.
- Rent, groceries and utilities are entered once and held flat in real terms for the whole horizon. Rent is usually the biggest line item and, in tight housing markets, tends to rise faster in real terms than groceries or utilities on lease renewal. If that holds here, the couple's larger shared-rent line grows the synergy surplus faster than modelled, so the nest egg shown likely understates what a couple staying together for decades would actually accumulate.
- No joint liability or tax-filing-status effects. A shared lease typically makes both partners liable for the full rent if one income drops, and marriage/registered-partnership status can change tax owed either way depending on jurisdiction. Neither is reflected in the synergy number.
- One-off costs of combining two households are excluded: one deposit instead of two is a saving not counted here, but a bigger shared place, duplicate furniture, or running two homes during a transition period are costs also not counted. The net effect on the true first-year cost could go either way.