The "no-buy" started as a TikTok challenge: pick something you spend too much on, clothes, make-up, takeaway food, and stop buying it for a set period, a month or a year. Its cousin, "underconsumption core", went round social media in the summer of 2024 with videos of ancient hairdryers and clothes worn since school. The label is new. The idea is not.
For anyone aiming at financial independence, the question is narrower: how many years does it buy? The answer turns almost entirely on a detail the videos skip. A no-buy year that ends on 31 December brings the date forward by a few months. The same cut in spending, if it becomes a habit, brings it forward by years.
The method, in one equation
The standard calculation rests on three assumptions, worth stating before reading any figures. You start from zero. Money saved earns 4% a year above inflation, the real return IgniFI uses for its own projections. And you are independent once your capital reaches 25 times your annual spending, which is the 4% rule: enough to fund what you spend today, without cutting it.
Call E the year's savings and D the year's spending. After n years the pot is worth E × (1.04ⁿ − 1) / 0.04. You reach the target when that equals 25 × D. Since 25 × 0.04 = 1, the equation tidies itself up: 1.04ⁿ = 1 + D / E. And 1 + D / E is income divided by savings, the inverse of the savings rate s.
So: n = ln(1/s) / ln(1.04).
Income has dropped out. At the same savings rate, someone earning €2,000 a month and someone earning €6,000 take the same number of years to reach their own target, because each target is built from their own spending. At a 10% savings rate it takes 58.7 years. At 50%, 17.7 years.
Starting from zero, with a target of 25 times annual spending and savings paid in at the end of each year. Real return of 4% (IgniFI's assumption) or 2% (a cautious one), in today's euros. Spending once independent is assumed equal to spending now. IgniFI calculation: n = ln(1 + 25 r (1 − s) / s) / ln(1 + r).
The curve is steep on the left and flat on the right. Going from 10% to 20% saves nearly eighteen years. Going from 60% to 70% saves four. That is why the first effort matters so much, and why the extreme frugality records posted online matter so little to most people.
With French figures
French households set aside 17.2% of their gross disposable income, according to INSEE for the second quarter of 2026, but only 8.5% as financial savings, most of the rest going into property. The median standard of living works out at €2,228 a month for a single person in 2024.
The table applies the formula to a take-home salary of €3,000 a month, about a third above that median. The years depend only on the rate; the euro amounts are there to help you place yourself.
| Savings rate | Saved per month | Spent per month | FIRE target (25 × annual spending) | Years at 4% real | Years at 2% real |
|---|---|---|---|---|---|
| 10% | €300 | €2,700 | €810,000 | 58.7 | 86.1 |
| 20% | €600 | €2,400 | €720,000 | 41.0 | 55.5 |
| 25% | €750 | €2,250 | €675,000 | 35.3 | 46.3 |
| 30% | €900 | €2,100 | €630,000 | 30.7 | 39.0 |
| 40% | €1,200 | €1,800 | €540,000 | 23.4 | 28.3 |
| 50% | €1,500 | €1,500 | €450,000 | 17.7 | 20.5 |
| 60% | €1,800 | €1,200 | €360,000 | 13.0 | 14.5 |
Starting from zero. Check on the 10% row: €3,600 a year for 58.7 years at 4% gives 3,600 × (10 − 1) / 0.04 = €810,000, since 1.04 to the power 58.7 is 10. INSEE, Informations rapides no. 212 (28 August 2026) and Insee Première no. 2117 (July 2026).
What a no-buy changes, month by month
Take a 32-year-old on that €3,000 salary. They spend €2,250, save €750, a 25% rate, and already have €40,000 invested. Their target is €675,000. They cut €300 a month of spending: clothes, online shopping, food delivery.
| Scenario | Saved per month | FIRE target | Age reached | Difference |
|---|---|---|---|---|
| Nothing changes | €750 | €675,000 | 62 years 11 months | |
| One no-buy year, then old habits | €1,050 for 12 months, then €750 | €675,000 | 62 years 7 months | 4 months sooner |
| The €300 cut becomes permanent | €1,050 | €585,000 | 55 years 6 months | 7 years 5 months sooner |
| The other way: €100 more spending, for good | €650 | €705,000 | 66 | 3 years 1 month later |
Monthly calculation, 4% real return a year, amounts in today's euros. The state pension is not counted.
The gap between the second and third rows is the whole story. The challenge year adds €3,600 of savings, which compound quietly and win four months. The habit works twice: it raises savings every month, and it lowers the target, because every euro of monthly spending removed takes €300 off the capital needed (1 × 12 × 25). Here, €300 less a month means €90,000 less to reach.
The higher savings do most of the work: on their own, they would bring the date forward by five years. The lower target on its own would bring it forward by two years and eight months. The two overlap a little and add up to seven years and five months.
Capital at the end of each year, in today's euros, for someone starting with €40,000 at 32, at a 4% real return. The two flat lines are each habit's FIRE target (25 times annual spending): the capital crosses them after about 23 and a half years in one case and 31 in the other. IgniFI calculation.
The limits, and they are real
The rebound. A year of going without that ends in a catch-up spree, a new wardrobe in January, wipes out the four months. The challenge only has financial value if it uncovers spending you are glad to have stopped.
Lifestyle creep. This is the opposite movement, and it is quieter. Every pay rise that goes entirely into extra comfort leaves the savings rate where it was, and every lasting €100 a month of extra spending adds €30,000 to the target. In the example, that €100 pushes the date back three years. Nobody decides to work three more years. People decide on a subscription, a slightly bigger car, a slightly dearer flat.
The social cost. Turning down dinners, weddings and weekends away with friends has a price that appears in no table. Some spending keeps up relationships that, over thirty years, matter more than a quarter's head start on a date.
Chosen frugality and imposed frugality. A lecturer at the London College of Fashion, quoted by the Belgian broadcaster RTBF in 2024, described underconsumption core as chosen sobriety, open to consumers who have something to choose from. In France, 15.4% of the population lived below the poverty line in 2024, which is €1,337 a month for a single person. For those households a no-buy is not a trend. It is the budget.
It is not a contest of virtue. The formula cannot tell a holiday from a coat. A 60% savings rate is not morally better than 20%; it is another way of splitting a life between now and later. The only useful question is whether the spending that remains is the spending you care about. And cutting spending has a floor where income has none: it is one of the four levers, not the only one.
Where IgniFI fits
IgniFI will not tell you what to stop buying. The app works out your real savings rate from your bank statements, category by category, and the budget page projects a FIRE date from the savings you actually made, at the same 4% real return used in this article. That is how you find out, three months in, whether a no-buy moved anything or simply shifted purchases from one month to the next. For the small expenses that add up, see also our piece on small expenses.
Sources
- INSEE, Informations rapides no. 212 (in French), 28 August 2026: household savings rate and financial savings rate in the second quarter of 2026.
- INSEE, Insee Première no. 2117, "Niveau de vie et pauvreté en 2024" (in French), July 2026.
- Boston University, Questrom School of Business, "No-Buy Challenge: does the TikTok trend really work?", June 2024.
- RTBF, "#Underconsumption Core ou quand consommer moins devient tendance" (in French), 2 September 2024.
- IgniFI calculations: closed formula for the chart and the first table, month-by-month simulation for the second.