Estimated from memory, before calculating the real number.
Once several months of real statements were reviewed directly.
From sizing a 3-month fund on the wrong, understated number.
Calculate the Number, Don't Estimate It
Charlie Munger's published circle-of-competence principle draws a hard line between genuine, verified understanding and a comfortable estimate, and circle of competence applied to knowing your own true monthly burn rate tests whether a household has actually calculated its essential monthly expenses from real records, or is simply estimating from memory. For example, consider a household that assumed $3,100 a month covered its essential expenses, sizing a 3-month, $9,300 emergency fund around that number. Reviewing six months of actual bank and card statements revealed the real essential burn rate was $3,750, a category the household hadn't fully counted including insurance premiums paid quarterly and a minimum debt payment easy to forget when estimating from memory. The properly sized 3-month fund should have been $11,250, a $1,950 gap the household never knew existed because it had estimated rather than calculated. Per Poor Charlie's Almanack, genuine understanding verified against real data was treated as fundamentally different from a comfortable, untested assumption. As of July 2026, this is especially important if your emergency fund's target size was based on an estimated monthly expense figure rather than one calculated from actual statements.
The gap between assumption and calculation quietly undersized the emergency fund by $1,950.
Calculate From Statements, Not Memory
Per the Berkshire Hathaway letter archive, Munger's published work consistently favored verified, specific data over a comfortable general impression. Keeping the properly sized fund in an account earning close to 4.20% APY, confirmed through FDIC deposit insurance resources, ensures the corrected fund also earns a competitive return.
| Approach | What it usually produces | Next check |
|---|---|---|
| Estimating essential expenses from memory | Often understated, missing infrequent bills | Review 3-6 months of actual statements |
| Calculating from real statements | A more accurate, often higher, number | Recalculate periodically as expenses change |
| Fund sized on an estimated number | Likely undersized relative to real need | Read a mental model for sizing your cash cushion |
| Fund sized on a calculated number | More reliably matched to actual need | Continue periodic recalculation |
Calculating your true burn rate has real benefits: it reveals the accurate basis for sizing an emergency fund, rather than trusting a comfortable but often understated estimate. The risk of estimating from memory, as the $1,950 gap example shows, is a real, undersized fund that feels adequate until an actual disruption reveals the shortfall. However, that said, it depends on how much your expenses have changed compared to when you last estimated: a household whose expenses have shifted, a new bill, a changed insurance premium, needs a fresh calculation more urgently than one with a genuinely stable pattern. If you're deciding whether to trust your current burn-rate estimate, choose to trust it if you've calculated it from actual statements within the past year; choose to recalculate if you're relying on memory or an old estimate. This is when this matters most: before an actual disruption tests whether your fund size was accurate.
Pull 3-6 months of actual statements rather than relying on memory.
Quarterly insurance and annual fees are easy to miss when estimating.
The multiple of months only works if the underlying number is accurate.
A new bill or changed premium shifts the true burn rate.
When This May Not Apply
A household that has recently calculated its burn rate directly from statements and confirmed its fund is sized accordingly doesn't need to redo the exercise until expenses meaningfully change. This is especially important to confirm rather than assume, since even a recent estimate can miss infrequent bills.
What to Do Next, in 20 Minutes
- Pull 3-6 months of actual bank and card statements.
- Calculate your true essential monthly expenses, including infrequent bills.
- Compare that number against your current emergency fund target.
- Read a mental model for sizing your cash cushion beyond a fixed number of months and deleveraging your own balance sheet for related frameworks.
- Read how big should your emergency fund actually be for sizing guidance.
- Run a full Money Map check to confirm your fund matches your real burn rate.
Sources and Methodology
This article applies Charlie Munger's published circle-of-competence principle to household burn-rate calculation. It is educational and does not constitute personalized financial advice.
- Poor Charlie's Almanack· Checked 2026-07-10
- Berkshire Hathaway letters· Checked 2026-07-10
- FDIC deposit insurance coverage· Checked 2026-07-10
- SwitchWize methodology· Checked 2026-07-10
Next scheduled verification: 2026-10-10
Educational content from the SwitchWize Research Desk. Charlie Munger and related entities are not affiliated with or endorsing SwitchWize.
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Disclaimer
This article is educational and does not provide personalized investment, tax, legal, or financial advice. Charlie Munger, the Munger estate, Berkshire Hathaway, and related entities are not affiliated with or endorsing SwitchWize. References to public letters, speeches, and books are used for educational interpretation only.