(Why a nuclear physicist’s trick is the ultimate life-hack for normal people)
You don’t need to split atoms to earn a living, but you can still steal Enrico Fermi’s super-power.In July 1945, Fermi stood 10 miles from the Trinity test — the world’s first nuclear explosion. While others were awestruck, he calmly tore a sheet of paper into strips, dropped them, and watched how far the shockwave pushed them. From that alone he estimated the yield was roughly 10 kilotons of TNT. He was off by less than a factor of 2. The official result came days later: 21 kt.That, my friends, is the art of the ball-park estimate — and it works for rent negotiations, retirement planning, and losing 10 kg just as beautifully as it works for mushroom clouds.Most of us face questions every year that feel impossibly big:
Fermi estimation sits in the sweet middle: 2–5 minutes, a napkin, and answers that are “right enough” to make confident decisions.Here are ready-to-use Fermi formulas for the questions that actually keep Indians awake at night (2025 edition).1. Retirement Corpus – The India 3.5–4 % RuleAnnual expenses you want in retirement × 28 ≈ Corpus needed today
Example: ₹80,000 monthly (today’s rupees) → ₹9.6 lakh/year → ₹2.7–3 crore corpus.2. Monthly Savings to Reach the CorpusCorpus needed ÷ (Years left × 140) ≈ Monthly SIP amount
(140 is the magic number for ~7.5 % real return over long periods in Indian equity-heavy portfolios)
Want ₹3 cr in 15 years? → ₹3 cr ÷ (15 × 140) ≈ ₹1.43 lakh/month starting today.3. Apartment Price Today (Tier-1 / good Tier-2 cities)Rough 2025 carpet rate: ₹9,000–15,000 per sq ft
1000 sq ft super-built-up (≈ 750–800 carpet) → ₹1.2–2.2 crore today
Next year? Add 12–15 % (appreciation + construction inflation) → ₹1.35–2.5 crore.4. Home Loan EMI Thumb RuleEMI ≈ Loan amount in lakhs × 90–100 per month (20-year loan @ 8.5–9.5 %)
₹1 crore loan → ₹90k–₹1 lakh monthly. Simple.5. Project Manpower (Software / any team work)Effective person-months available = Team size × Calendar months × 0.7 (70 % efficiency is realistic)
8 people × 6 months × 0.7 ≈ 33–35 person-months of actual work.
If your project historically needs 60 person-months, you’re doomed unless you add people or time.6. Hobby / Skill Time BudgetMost skills: 400 ± 200 hours to reach “pretty decent” (conversational language, decent guitar, intermediate coding, etc.)
Want conversational Spanish in one year? 400 hours ÷ 300 workable days ≈ 80 minutes/day
(or 2–3 hours on weekends only and chill on weekdays).7. Weight Loss – The Lazy Math1 kg fat ≈ 7700 calories
10 kg in 365 days = 210-calorie daily deficit on average
Realistic combo: Eat 300 cal less + walk 7–8k extra steps (≈ 300 cal) → 1 kg every 12–14 days → 10 kg in a year without starving.The 5-Step Fermi Recipe You Can Use While Drinking Chai
You don’t need one in 2025.Grab a napkin. Pick one question that’s been nagging you.
I’ll wait here while you become 1 % as cool as Enrico Fermi.(And if you want, drop your question in the comments — we’ll Fermi it together.)— Someone who stopped guessing and started estimating
Bangalore, December 2025
You don’t need to split atoms to earn a living, but you can still steal Enrico Fermi’s super-power.In July 1945, Fermi stood 10 miles from the Trinity test — the world’s first nuclear explosion. While others were awestruck, he calmly tore a sheet of paper into strips, dropped them, and watched how far the shockwave pushed them. From that alone he estimated the yield was roughly 10 kilotons of TNT. He was off by less than a factor of 2. The official result came days later: 21 kt.That, my friends, is the art of the ball-park estimate — and it works for rent negotiations, retirement planning, and losing 10 kg just as beautifully as it works for mushroom clouds.Most of us face questions every year that feel impossibly big:
- How much money do I actually need to retire?
- Can I afford that flat next year?
- How many people do I need to finish this project in six months?
- How much time should I spend on the guitar to not suck in a year?
Fermi estimation sits in the sweet middle: 2–5 minutes, a napkin, and answers that are “right enough” to make confident decisions.Here are ready-to-use Fermi formulas for the questions that actually keep Indians awake at night (2025 edition).1. Retirement Corpus – The India 3.5–4 % RuleAnnual expenses you want in retirement × 28 ≈ Corpus needed today
Example: ₹80,000 monthly (today’s rupees) → ₹9.6 lakh/year → ₹2.7–3 crore corpus.2. Monthly Savings to Reach the CorpusCorpus needed ÷ (Years left × 140) ≈ Monthly SIP amount
(140 is the magic number for ~7.5 % real return over long periods in Indian equity-heavy portfolios)
Want ₹3 cr in 15 years? → ₹3 cr ÷ (15 × 140) ≈ ₹1.43 lakh/month starting today.3. Apartment Price Today (Tier-1 / good Tier-2 cities)Rough 2025 carpet rate: ₹9,000–15,000 per sq ft
1000 sq ft super-built-up (≈ 750–800 carpet) → ₹1.2–2.2 crore today
Next year? Add 12–15 % (appreciation + construction inflation) → ₹1.35–2.5 crore.4. Home Loan EMI Thumb RuleEMI ≈ Loan amount in lakhs × 90–100 per month (20-year loan @ 8.5–9.5 %)
₹1 crore loan → ₹90k–₹1 lakh monthly. Simple.5. Project Manpower (Software / any team work)Effective person-months available = Team size × Calendar months × 0.7 (70 % efficiency is realistic)
8 people × 6 months × 0.7 ≈ 33–35 person-months of actual work.
If your project historically needs 60 person-months, you’re doomed unless you add people or time.6. Hobby / Skill Time BudgetMost skills: 400 ± 200 hours to reach “pretty decent” (conversational language, decent guitar, intermediate coding, etc.)
Want conversational Spanish in one year? 400 hours ÷ 300 workable days ≈ 80 minutes/day
(or 2–3 hours on weekends only and chill on weekdays).7. Weight Loss – The Lazy Math1 kg fat ≈ 7700 calories
10 kg in 365 days = 210-calorie daily deficit on average
Realistic combo: Eat 300 cal less + walk 7–8k extra steps (≈ 300 cal) → 1 kg every 12–14 days → 10 kg in a year without starving.The 5-Step Fermi Recipe You Can Use While Drinking Chai
- Write the scary question in one sentence.
- Break it into pieces you kind-of know (or can reasonably guess).
- Say every assumption out loud (“8 % inflation”, “12 % property rise”, “7 kg fat = 54,000 cal”, etc.).
- Multiply and divide brutally. Rounding to nearest round number is encouraged.
- Sanity check: Does the answer feel absurd? Change one assumption and see how much it moves.
You don’t need one in 2025.Grab a napkin. Pick one question that’s been nagging you.
I’ll wait here while you become 1 % as cool as Enrico Fermi.(And if you want, drop your question in the comments — we’ll Fermi it together.)— Someone who stopped guessing and started estimating
Bangalore, December 2025