A business plan assumptions sheet is a one-page table that lists every important number in your plan, where it came from, how sure you are of it, and how you could check it, so nobody (including you) has to wonder which figures are facts and which are guesses.
Lenders, partners, and investors may read a plan differently than its author does. The author sees a story. A reviewer may see a stack of claims and keep asking, “Where did that number come from?” If you can answer instantly, the whole plan feels more solid. If you can’t, even sensible figures start to look like decoration.
This guide shows how to build an assumptions sheet from scratch, how to use AI to pull every number out of a draft and check it for missing support (without letting it invent anything), how to rank your confidence honestly, and how to turn weak spots into a to-do list. It uses an invented home bakery as the example, and all figures are made up.
For a general overview of what financial assumptions are, see Chron’s explanation of financial assumptions in a business plan. The method below goes a step further by attaching evidence and confidence to each one.
Contents
What Counts as an Assumption
An assumption is any number or claim in your plan that you haven’t proven yet. Common ones include:
- Prices you plan to charge
- Sales volume per month or per season
- Costs you expect to pay, such as rent, ingredients, and marketing
- Growth rates from month to month
- Customer behavior, such as how often people reorder
- Timing, such as when you’ll open or when you’ll have your first customers
- Market facts, such as the number of potential customers in your area
Some are easy to verify, like a signed lease. Others are only educated guesses, like how many customers you’ll get in month three. A good plan treats these differently, and the assumptions sheet makes the difference visible.
Three Ways to Handle Your Numbers
| Numbers scattered in the plan | A list of figures | An assumptions sheet | |
|---|---|---|---|
| Shows where each number came from | No | Rarely | Yes |
| Shows how sure you are | No | No | Yes |
| Helps you answer lender questions | Poorly | Somewhat | Directly |
| Shows what to verify next | No | No | Yes |
In my view, the sheet takes some effort once and pays off every time someone asks you to defend a number.
Think of a business plan assumptions sheet as the receipts for your plan: the plan makes the claims, and the sheet shows what stands behind them.
Build the Sheet: Columns, Confidence, and Sources
The Columns
A simple sheet needs only these columns:
- Assumption: What the number is (for example, “Price per box”)
- Base value: Your main estimate
- Low / High: A realistic range, when the number is uncertain
- Source: Where it came from (quote, contract, record, own test, article with date)
- Date: When you got the information
- Confidence: Confirmed, Estimated, or Unknown
- How to verify: The next step to firm it up
- Used in: Where this number appears in the plan
The “How to verify” column is what makes the sheet useful beyond the plan. It turns uncertainty into tasks.
Three Levels of Confidence
Keep it simple. Three levels are enough.
- Confirmed: You have a document or a record. Examples: a signed lease, a supplier quote with a date, your own sales records, a fee schedule published by a payment provider.
- Estimated: You reasoned your way to it from evidence, and you can explain how. You also have a range. Examples: a price based on checking several competitors, an ingredient cost based on one batch.
- Unknown: You have no evidence yet. It’s a placeholder or a hope. Examples: how many customers will reorder, how fast word of mouth will spread.
Be honest about the third category. An honest “Unknown” with a plan to find out is, in my view, stronger than a confident number with nothing behind it.
Ranking the Quality of Sources
Not all sources carry equal weight. My suggested order, from strongest to weakest:
- Your own records: invoices, bank statements, sales history
- Written quotes or contracts from suppliers, landlords, or insurers
- Your own tests, such as a pre-order run, a pop-up sale, or a small pilot
- Published data from reliable organizations, with the date and link recorded
- Comparisons with similar businesses, noted as comparisons
- Personal judgment, clearly labeled as such
Higher on the list means you can defend it more easily. When you notice a key number resting on source 6, that’s your cue to look for a stronger source.
Use AI to Pull Every Number Out of Your Draft
Once you have a draft plan, or even just a few paragraphs, AI can do the tedious part, which is finding each number and putting it into a list. This prompt keeps it from adding anything:
Below is a draft of my business plan.
[paste plan text, with any private details removed]
Please do the following using ONLY what is written above:
1. List every number or numeric claim in the plan (prices, volumes, costs, growth rates, dates, market claims).
2. For each one, write: the number, the sentence it appears in, and what it relates to.
3. For each, mark whether the plan itself gives a source or evidence. Use "Source stated," "No source stated," or "Unclear."
4. List numbers that seem to conflict with each other, and explain why.
5. List important numbers that are NOT in the plan but would normally be needed (as questions only).
Rules:
- Do not add or estimate any numbers.
- Do not tell me whether numbers are realistic.
- If a number appears more than once with different values, list every version.I wrote this prompt myself as a suggestion; no source publishes it as a standard.
You’ll get a list of every figure and a flag for the ones with no support. Copy that into your sheet as the starting rows.
Fill In the Sheet Yourself
Now go through the rows and fill in the columns. Some are quick. For example, if your plan says “kitchen rental: $1,200 per month,” and you have a written rental agreement, the source is the agreement, the date is the signing date, and confidence is Confirmed.
Others take more work. If your plan says “We’ll sell 220 boxes a month by month six,” that’s a guess unless you have evidence. Write the reasoning, a range, and the label.
Don’t let AI fill these columns for you. The source and confidence are statements about what you know, and only you know that.
A Sample Sheet With Made-Up Entries
Here’s an invented sheet for the bakery.
| Assumption | Base | Range | Source | Date | Confidence | How to verify |
|---|---|---|---|---|---|---|
| Price per box | $28 | $26 to $30 | Price check, pre-order test | Sept 12 | Estimated | Second test at $30 |
| Ingredients per box | $6.40 | $6.00 to $7.20 | One real batch, receipts kept | Sept 20 | Estimated | Second supplier |
| Kitchen rental | $1,200/mo | None | Signed agreement | Aug 30 | Confirmed | None needed |
| Marketing budget | $450/mo | $300 to $600 | Own plan | Sept 25 | Estimated | Test one ad |
| Boxes sold per month | 220 | 120 to 260 | Pop-up sales | Sept 28 | Unknown | Track weekly sales |
| Repeat customer rate | 40% | Unknown | No source | — | Unknown | Ask customers |
| Payment processing | $1.20/box | None | Fee schedule | Sept 18 | Confirmed | Recheck on changes |
Look at what the sheet tells you at a glance. The rent is solid. The price and ingredient cost have some support. The sales volume and repeat rate are the weak points, and they’re also the numbers the whole plan depends on most. Now you know where to spend your effort, and you know what a lender may ask about first.
Link the Sheet to Break-Even and Projections
Once you have your assumptions in one place, you can use them as inputs in a break-even calculation or a monthly projection. The key rule: every number in the projection should link back to a row in the sheet. If a number isn’t on the sheet, it shouldn’t be in the plan.
This also prevents a common problem, where the same figure appears in three places with slightly different values. When a number changes, you change it once in the sheet and update the plan.
What the SBA Expects Behind the Numbers
The SBA: Write your business plan says financial projections should support your funding request by showing the business is stable and likely to succeed. For a new business it describes a five-year outlook with forecasted income statements, balance sheets, cash flow statements and capital expenditure budgets, with year one broken into quarterly or even monthly projections. It also says to explain the projections and match them to your funding request, and that lenders compare expected costs to projected revenue to judge whether the business can profit. Every assumption in your sheet is one of the inputs to those statements, which is why each one needs a source and a confidence level.
Stress-Test the Sheet and Fix Weak Spots
Practice the Questions a Reviewer Might Ask
With your sheet ready, you can rehearse the conversation. Ask AI to play a reviewer, using only your sheet:
Act as a reviewer reading my assumptions sheet.
[paste sheet]
Ask me the five questions you would ask first about these numbers, focusing on rows where confidence is Estimated or Unknown. For each question, quote the row that prompted it.
Rules:
- Use only what is in the sheet.
- Do not suggest what my answers should be.
- Do not give opinions about whether my business will succeed.Then write brief answers in your own words. For “How many boxes will you sell?” your answer might be: “That’s currently an estimate based on pop-up sales. I’m tracking weekly sales and will update the plan after two months. At the low end of my range, I’d need to cut marketing and rely on repeat orders.”
That sounds like someone who knows their numbers, which is the whole point.
Turn Weak Spots Into a To-Do List
Sort the sheet by confidence and look at the Unknown and Estimated rows. For each, choose the cheapest way to get better evidence:
- Ask for a written quote from suppliers
- Run a small test, such as a weekend pop-up or a pre-order
- Survey a handful of likely customers with a simple question
- Check published data from a reliable organization and record the link and date
- Look at your own records if you’ve been selling informally
Then update the sheet and change the confidence level when the evidence comes in. Over a few weeks, the sheet shifts from guesses to facts.
Handling Numbers From Articles and Reports
When you use outside data, record the exact source, the date, and the definition. A figure like “market size” can mean many things, and it often depends on how the author defined it. Write down what exactly was measured, for what area, and for which year. If you can’t tell, mark it as Unclear.
Treat vendor claims and promotional articles with extra caution. If the only source for a number is a company that sells something related, look for a second, independent source.
Housekeeping and Common Mistakes
Keep Versions and Notes
Save a dated copy each time you make major changes, such as “Assumptions_2026-10-05.” Add a short note explaining what changed and why. When someone asks why your price assumption moved from $26 to $28, you can show the test that changed your mind.
Protect Private Information
Before you paste a plan into any AI tool, remove personal details, addresses, account numbers, and anything your partners or customers wouldn’t want shared. You can replace names with placeholders. The sheet works fine without them.
Business Plan Assumptions Sheet: Mistakes to Avoid
- Hiding guesses inside polished text. Label them instead.
- Treating a range as a single number. Show low, base, and high.
- Listing a source without a date. Information goes stale.
- Filling every row with “Estimated.” Some are truly unknown, and saying so builds trust.
- Never updating the sheet. It should change as you learn.
- Letting AI choose the values. AI can extract and organize, but the facts must come from you.
Questions About Business Plan Assumptions Sheet
How long does a business plan assumptions sheet take to build? It depends on how many numbers your plan contains; most of the work goes into finding sources, and later updates only touch the rows that change.
When should I create a business plan assumptions sheet? As early as you have numbers in a draft. Building it before you polish the plan means weak spots show up while they’re still easy to fix.
Do I have to share the sheet with lenders? Not necessarily. You may keep it as a backup so they can answer questions. Sharing it can also show that you’ve done your homework, depending on the situation.
How many assumptions should it include? Include the ones that drive your results: price, volume, main costs, timing, and key customer behaviors. For a small business, I would keep it to ten to twenty rows, which is my own rule of thumb rather than a standard.
What if most of my rows say Unknown? That can be normal at the start. The sheet’s job is to show you what to find out next. Begin with the ones that most affect your results.
Can AI fill in the sources for me? It can’t know where your information came from. It can help you list questions about sources, but the facts and the dates have to be yours.
Is this a replacement for professional advice? No. For loan applications, tax matters, or legal terms, consult a qualified professional.
Start With the Five Numbers That Matter Most
Pick the five numbers your plan depends on most, such as price, monthly sales, main cost, marketing spend, and start date, and give each a source, a date, a range, and a confidence level. A business plan assumptions sheet doesn’t make your numbers better by itself. It makes them honest, and, in my view, honest numbers are easier to improve, easier to defend, and less likely to surprise you in the middle of a conversation.
This article offers general educational information, not financial, tax, or legal advice. Examples use fictional numbers.
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