Interviewers almost never tank a candidate for taking twelve seconds instead of eight. They tank the ones who go quiet, round twice in opposite directions, or land on a number with no units and no so-what. Speed is a byproduct of having four reusable moves; it is not the skill itself.
That is the thesis of this piece. If you can run percent anchors, growth and CAGR approximations, a one-line breakeven, and a rounding discipline you can defend out loud, the arithmetic in a first-round case stops being the scary part. The rest of the page teaches those moves with numbers you can reproduce, cites what the firms themselves publish about calculator-free work, and then points at the free-forever drill Ned built so you can time the same loops.
What the firms actually say about calculators
Firms publish less about live case math than prep folklore invents, but they are clear on one point: when calculation shows up in an interview, you own it without a device.
McKinsey's Problem Solving Test practice booklet tells candidates not to use electronic devices (explicitly including a calculator) while answering, and notes devices are not permitted in the real administration. That is a screening test, not a partner case, but it is McKinsey in writing: quantitative judgment under time, by hand.
Bain's digital assessment page is even more explicit about the interview boundary. For Test Gorilla and Sova it allows a pen, paper, and calculator, then adds the line candidates should remember: calculators are allowed for the digital assessment only, not during any subsequent interviews. The screen may let you punch buttons; the case does not.
BCG's case interview preparation page lists "perform quick calculations" as part of the live problem-solving conversation, alongside structuring and judgment. On the France careers page, a BCG interviewer advises candidates to sanity-check mid-calculation and, especially on remote interviews, to speak the quantitative steps out loud, because the reasoning matters as much as the final figure. CoachNed is not affiliated with McKinsey, BCG, Bain, or any firm named here; use their pages as the source of truth for their own processes.
| Setting | Calculator | What you should practice |
|---|---|---|
| Bain digital assessment (Sova / Test Gorilla) | Allowed per Bain | Timed reading of tables; still narrate for the interview that follows |
| McKinsey PST-style screening (historical practice PDFs) | Not allowed | Hand arithmetic, scratch in the booklet, no device |
| Live case / problem-solving interview | Effectively none | Equation aloud, units, one defensible round, so-what |
The implication for prep is narrow. Drill the hand-math you will need when the calculator is gone. Do not confuse a digital screen that permits a calculator with the room that does not.
Four moves that cover most case arithmetic
Almost every first-round number question collapses into one of four patterns. Memorize the pattern, not a sheet of formulas.
1. Percent anchors (10%, 1%, then build)
Move the decimal once for 10%. Move it twice for 1%. Build the rest by addition and subtraction.
Worked example. Harbor Metals asks: what is 17% of $84m revenue?
- 10% of 84 = 8.4
- 5% = half of 10% = 4.2
- 2% = twice 1% = 1.68
- 17% = 10 + 5 + 2 = 14.28, so about $14.3m
Say the build out loud. The interviewer can follow you; if you mistype a digit they can still salvage the path. The flip shortcut is the sibling move: 8% of 25 equals 25% of 8, which is 2. Use whichever base is cleaner.
2. Growth and CAGR approximations
Single-period growth is (new − old) / old. Compound growth over several years is where candidates invent precision they cannot compute. Use the Rule of 72 for doubling, then interpolate.
Worked example. Northline Logistics revenue went from $40m to $80m in six years. What CAGR should you quote?
Doubling in six years implies roughly 72 / 6 ≈ 12% per year. The exact fifth-root or sixth-root calculation is not what the sheet rewards; a clean "about 12%" with the rule named is. If volume rose 4% and price rose 3% in the same year, revenue is about 7% up for small rates (true compound is 7.12%; say "roughly seven" and move on).
Forward projection for low rates: 5% for three years is about +15%, so end ≈ 115% of start. True compound is ~115.8%. Close enough to decide whether a plant pays back.
3. Breakeven in one line
Units to break even = fixed cost / contribution per unit. Contribution = price − variable cost. Write that sentence before any division.
Worked example. Fixed cost $2.4m. Price $50. Variable $20. How many units?
Contribution = 30. Units = 2,400,000 / 30 = 80,000. If the interviewer then shocks price to $45, contribution becomes 25 and breakeven jumps to 96,000. The skill is reusing the one-line setup, not memorizing 80,000.
4. Rounding discipline
Round once, early, and say by how much. Never round intermediate steps in opposite directions and then treat the final digit as sacred.
Worked example. 487 stores × $312 average ticket.
Round to 500 × 300 = 150,000, and say "I rounded stores up about 3% and ticket down about 4%, so the product is within a few percent of true." True is about 151,900. A candidate who rounds 487 to 490, then 312 to 310, then multiplies silently, looks slower and less trustworthy than the person who announced the approximation.
Ned's rule. If you cannot name the equation in one sentence before the first digit, you are not doing case math yet. You are gambling. The sheet rewards a visible setup more than a lucky final integer.
A full mini-case you can time
Run this with a phone timer. Ninety seconds for the equation, ninety for the number, thirty for the so-what. No calculator.
Facts. Cedar Peak Outdoor sells jackets. Price $180. Variable cost $72. Fixed annual overhead $9.6m. Last year they sold 95,000 jackets. A channel partner wants a 12% price cut in exchange for guaranteed volume of 120,000 jackets. Ignore cannibalization of other products. Should Cedar Peak take the deal on contribution alone?
Equation (say it). Contribution today = (180 − 72) × 95,000. Contribution under the deal = (180 × 0.88 − 72) × 120,000. Compare the two totals to fixed cost only if you need profit; for the decision, compare contribution.
Math. Current contribution per jacket = 108. Total = 108 × 95,000. 100 × 95k = 9.5m; 8 × 95k = 760k; total $10.26m.
New price = 180 × 0.88. 10% of 180 = 18; 2% = 3.6; cut = 21.6; new price = 158.4. Contribution per = 158.4 − 72 = 86.4. Total = 86.4 × 120,000. 86 × 120k = 10.32m; 0.4 × 120k = 48k; total $10.368m.
So-what. Contribution rises by about $100k while volume jumps 26%. On contribution alone the deal is slightly better, but the margin per jacket falls from 108 to 86.4 (about a 20% drop in unit contribution). I would take it only if the partner volume is contractual and Cedar Peak has spare capacity; otherwise negotiate a smaller cut or a volume floor with a clawback.
That last paragraph is what interviewers are listening for. The arithmetic is table stakes; the judgment is the score.
What CoachNed Quick Math actually is
Ned built Quick Math (also reachable at /quick-math) as a free-forever arithmetic gym. It is not a full case and not an AI critique of your prose. It is equation-first practice with an immediate check.
As of the live product:
- Three tracks: a guided warmup (five problems, no clock, see the move after each), a case sprint (five minutes of percent change, contribution, share, breakeven-style items), and a speed sprint (five minutes of short arithmetic to rebuild reflex).
- Each item shows a hint on request and, after you submit, the correct number plus the move.
- The lobby copy is blunt: name the equation out loud, keep units in the answer, and treat the move — not the streak — as the skill.
- A link out to the fuller math drills when you want written structure around the number, not only the digit.
It stays free forever, unlimited, and never locks. Everything else on CoachNed is open for seven days with no card; then $120 for a recruiting season or $49 a month. Quick Math does not burn that week. Use it as the daily warm-up; use a voice or guided case when you need the full interview arc.
How to place five minutes in a prep week
| Day | Minutes | Focus |
|---|---|---|
| Mon | 5 | Guided warmup; say every equation aloud |
| Tue | 5 | Case sprint; log which family you miss |
| Wed | 15 | One AI-graded math drill or a short case |
| Thu | 5 | Speed sprint if reflex is soft; else case sprint again |
| Fri | 18 | Full voice case or typed first rep |
If accuracy falls as you chase the clock, stay on the guided warmup until the move is automatic. If accuracy is stable above roughly 70% on a case sprint, the bottleneck has moved to narration inside a real case, not to the arithmetic gym.
Practice this today
Open /drills/quick-math. Run the guided warmup once without a timer. For every miss, rewrite the move on paper in your own words. Then sit one case sprint. Tomorrow, repeat only the family that hurt you. When the arithmetic feels dull, transfer to /drills/math or a full case so the number has to earn a recommendation.
Frequently asked questions
Is CoachNed Quick Math really free forever?
Yes. The live product states it is free forever, unlimited, and never locks. It sits at /drills/quick-math (and /quick-math). The paid plans cover the rest of the platform after the seven-day open week.
How is Quick Math different from the Case Math drill?
Quick Math checks a number under a short loop and shows the move. The Case Math drill is the fuller interview skill: set-up, units, and business meaning with AI grading. Warm up on Quick Math; transfer on the drill or a live case.
Do consulting firms allow calculators in case interviews?
Treat live interviews as calculator-free unless your invitation says otherwise. Bain states calculators are allowed on its digital assessment only, not in subsequent interviews. McKinsey's PST practice materials forbid electronic devices for that screen. Confirm the instructions for your office and format.
What should I practice if I only have ten minutes a day?
Five minutes of Quick Math on the family you miss most, then five minutes rewriting one missed move and saying a so-what sentence for a fake client. Volume without that second half trains mute arithmetic.
Who built Quick Math?
Ned — Edward "Ned" Calder, the Big Four Strategy & Operations partner behind CoachNed — built the drill so candidates could rehearse calculator-free case arithmetic without burning a paid case slot. The rest of the product (cases, voice interviews, assessment sims) sits around that habit.
Sources
- McKinsey Problem Solving Test practice test A — no electronic devices or calculators on the practice and actual PST administration. Checked 2026-09-24.
- Bain digital assessment — calculator allowed on Sova / Test Gorilla only; not during subsequent interviews. Checked 2026-09-24.
- BCG case interview preparation — live case includes quick calculations as part of the problem-solving conversation. Checked 2026-09-24.
- BCG France: the case interview — interviewer guidance to sanity-check and speak quantitative steps aloud. Checked 2026-09-24.
- McKinsey interviewing overview — problem-solving interview as a typical client scenario; firm prep resources. Checked 2026-09-24.