What it means
Finance is full of shortcuts that busy people use to get to a sensible ballpark figure. A rule of thumb packages years of experience or simple maths into a single sentence that you can apply in your head during a meeting.
Examples include the Rule of 72 for doubling time and the guideline that a mortgage payment should not exceed about 28% of gross income. The strength of these rules is speed and shared language.
A founder who says "we should keep six months of cash" is using a rule of thumb that everyone in the room understands immediately. It sets a starting point for discussion without a spreadsheet.
The weakness is that every rule of thumb hides assumptions. The Rule of 72 is close for moderate interest rates but drifts for very high or very low ones, and a six-month cash buffer is far too small for a business with lumpy, seasonal income.
A rule that works for the average case can mislead you badly in an unusual one. In practice, finance teams use rules of thumb to screen ideas and spot errors.
If a model says a small shop will earn a 90% profit margin, a quick comparison with the normal range for retail suggests something is wrong before anyone builds a detailed forecast. The shortcut flags where to look, and the detailed analysis then confirms or corrects it.
A good habit is to treat a rule of thumb as a question rather than an answer. Ask what it assumes, when it breaks down and what the real figures say, then use the rule only as a cross-check on your fuller work.
In practice
Real-world examples.
Example
A restaurant owner uses the guideline that food costs should be about 30% of menu sales. On $40,000 of monthly sales she budgets $12,000 for ingredients, and when invoices come in at $15,000 she knows to investigate waste or supplier prices.
Example
A sales director estimates that marketing spend should be roughly 10% of revenue for a growing software company. With a $2,000,000 revenue plan he pencils in $200,000 for marketing before the detailed campaign plan is written.
Example
A couple planning a home purchase use the 28% housing guideline. With a combined gross income of $120,000 a year, they aim for housing payments of no more than $33,600 a year, or $2,800 a month.
Formula
Calculation
A rule of thumb is a general idea rather than one formula, but the best-known example is the Rule of 72: Years to double = 72 / Annual interest rate (as a whole number).
If an investment earns 8% a year, it doubles in about 72 / 8 = 9 years. Starting with $50,000, it would be worth about $100,000 after 9 years. The exact figure is a little different: $50,000 x 1.08 compounded for 9 years is about $99,950, so the shortcut lands within a fraction of a per cent of the true answer.Case study
Seen in the real world.
Brightwater Cafes is an entirely fictional chain of six coffee shops whose owner relied on one rule of thumb: rent should be no more than 10% of sales. The rule had served him well in busy city-centre sites, so he applied it without question when evaluating a seventh location.
The landlord's quoted rent was $5,000 a month, and the owner's sales forecast of $60,000 a month passed the test easily. His finance manager, in this illustrative story, then ran the full numbers and found that staff costs at that site would be higher because of longer opening hours, which pushed the shop to a loss.
The owner kept using the rent guideline as a first filter but added a one-page profit check for every new site. The illustrative lesson is that a rule of thumb earns a place at the start of a decision, not at the end.
Watch out
Common mistakes.
- Treating a rule of thumb as a law, and ignoring the circumstances in which it stops working.
- Applying a rule built for one industry or country to a completely different business without checking it.
- Using a rule of thumb in place of a proper calculation when the stakes are high, such as signing a lease or raising debt.
Questions
People also ask.
Where do rules of thumb come from?
They usually come from long practical experience or simple maths, and are passed on because they work well in typical cases.
Are rules of thumb accurate?
They are approximate by design, and are often close enough for screening decisions but not for final ones.
Should I use a rule of thumb in a financial model?
Use it to sanity-check results or set a rough starting assumption, and replace it with real data as soon as you have it.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%Related