How to Build a 13-Week Cash Flow Forecast in One Hour (Contractor’s Guide)

Every Thursday night, somewhere in Texas, a contractor opens his banking app and holds his breath.

I’ve watched this happen more times than I can count. Not long ago it was the owner of an HVAC company — mid-thirties, three crews, phone ringing off the hook, revenue north of half a million a year. By every outside measure, a business doing well. (I’ve had a version of this conversation with enough owners that you can consider him a composite. The numbers change. The look on their faces doesn’t.)

He told me the same thing I hear from general contractors, plumbers, electricians, and roofers: “I never actually know how much cash I have.” Not what the P&L says. Not what QuickBooks says. What’s really available, this week, after payroll clears and the supplier draft hits.

So he checks the bank balance before lunch. Then again before bed. And Friday still feels like a coin flip.

If that’s you, here’s the good news: this is one of the most fixable problems in small business finance. The fix is called a 13-week cash flow forecast, you can build the first version in about an hour, and you don’t need to buy any software to do it.

Why profitable contractors still run out of cash

First, let’s clear something up, because almost every owner I meet has quietly decided he’s “bad with money.”

You’re not bad with money. Your business outgrew the way you track it.

In the trades, the gap between doing the work and getting paid for the work is brutal. You finish the install in March. You invoice in March. The check shows up in May — if the GC pays on time, and if retainage isn’t holding another piece hostage. On commercial work, waiting 45 to 90 days for money you’ve already earned is normal, not an exception.

Meanwhile, payroll goes out every single Friday. Fuel, insurance, truck payments, and material suppliers don’t wait for your receivables.

That’s how a genuinely profitable company ends up short on a Tuesday. Making money and having money are not the same thing. Profit is an accounting opinion. Cash is a fact. A bank balance can’t tell you anything about next week, and next week is where the trouble lives.

What a 13-week cash flow forecast actually is

A 13-week cash flow forecast is a simple table — a ledger sheet or an Excel spreadsheet works fine — with 14 columns: one for the week you’re in, and one for each of the next 13 weeks. That’s one quarter, which is about as far as anyone can predict cash with a straight face.

Each week answers four plain questions:

• What do I start the week with?
• What’s coming in?
• What’s going out?
• What’s left at the end?

Whatever’s left becomes the starting number for the next column. That’s the whole machine. No finance degree, no dashboard subscription, no consultant required.

Why weeks and not months? Because cash problems are timing problems. A month that looks fine in total can contain one week where payroll, a loan payment, and an insurance premium all land together while your biggest customer is still “processing” your invoice. Monthly views hide that week. Weekly views light it up while you still have time to act.

How to build yours in about an hour

You don’t need a perfect system. You need a real one you’ll actually keep. Here’s the process I walk owners through.

1. Find your real number

Log into every business bank account and write down what’s actually there today. Not what you think is there. This is your starting line, and it’s the only number in the whole forecast you know for certain.

2. List everything coming in

Open invoices, scheduled draws, maintenance contracts, financing you’ve already closed. For each one, write down the week the money will realistically land — not the week the invoice says it’s due. If a customer has a habit of paying two weeks late, plan for two weeks late.

3. List everything going out

Payroll with taxes and benefits included, rent, suppliers, subs, loan payments, insurance, fuel cards, and the quiet auto-charges everyone forgets about. Go through two or three months of bank and credit card statements to catch the subscriptions hiding in there. Note the day each one leaves.

4. Build this week first

Fill in the current week’s column with what you actually know. Starting cash, plus what’s coming in, minus what’s going out, equals what’s left. Congratulations — you now know more about your cash position than most business owners ever do.

5. Stretch it 13 weeks out

Carry the pattern forward. Recurring items repeat on their schedule. One-time items go in the week they belong to. It gets rougher the further out you go, and that’s fine. A rough headlight beats driving in the dark.

6. Circle the tight weeks

Pick a cushion — the minimum you ever want left at the end of a week. Any week that dips below it gets circled. That circle is the entire point of the exercise: it’s a warning you got weeks in advance instead of on payday morning.

7. Set the 15-minute weekly review

Same day, same time, every week. Update what actually happened, add a new week at the far end, and glance at what changed. Setup takes an hour. Maintenance takes 15 minutes. The habit is what turns a spreadsheet into peace of mind.

See what a tight week looks like

Numbers on a page are one thing. Watching a late payment ripple through your quarter is another. In the example below, one customer owes you $10,000 and keeps saying “next week.” Drag the slider and watch what his delay does to your cash.

Notice what just happened. Nothing about your business changed — same jobs, same costs, same revenue. Only the timing moved. And the forecast showed you the pinch three weeks before it arrived, which is the difference between a calm phone call today and a payroll scramble later.

When you’re guessing, guess against yourself

One rule makes the whole forecast trustworthy: optimism is not your friend here.

Assume the customer who said “next week” means two weeks. Assume retainage gets released late. Assume the bills hit on their earliest possible day. It sounds gloomy, but it’s the opposite — planning around the slower, tighter version means bad weeks show up on your screen early, while you still have choices. If the money comes in faster than you planned, you get a pleasant surprise instead of a crisis. That’s the right direction to be wrong in.

And when a tight week does appear, you have time to work the problem. Pull out a sheet of paper and write down three to five ways to free up or pull in cash. Call the customer who owes you for that install from four weeks ago. Ask a supplier for another two weeks. Shift a non-urgent equipment purchase. None of these are fun calls, but every one of them is easier three weeks early than three hours before payroll.

Build the rainy-day cushion

The forecast handles week-to-week timing. But one blown transmission, one warranty claim, one slow-paying GC can still wipe out a quarter’s worth of careful planning. So the second piece of the system is a safety net, and it takes three steps:

• Open a separate sub-account. Ask your banker — it takes ten minutes. Money you can see mixed in with operating cash is money you will eventually spend.
• Set an automatic rule. Move 10% of every payment you receive into that account. Not “whatever’s left at the end of the month” — a fixed percentage, on autopilot, before you can think about it.
• Decide the rules before you need them. What balance are you building toward? A useful target for a trades business: enough to replace a service truck, or three months of fixed expenses, whichever number scares you less. And write down what qualifies as a real emergency, because the definition gets very flexible at 11 p.m.

Keep it alive without doing it all yourself

Here’s the honest weakness of this system: it only works while it’s current. The moment the forecast goes stale, you’re back to guessing, and the old Thursday-night feeling comes back with it.

So once you’ve built it and run it for a few weeks — long enough to understand it — hand off the data entry. Teach your bookkeeper to fill in the current week’s actuals, and give that task a deadline, like every Monday by noon. Your job shrinks to the 15-minute review: check the current week, scan the next 13, circle anything that turned orange.

You built it, you understand it, you review it. Someone else maintains it. That’s the version that survives busy season.

When to bring in a pro

Everything above, you can do yourself, this week, for free. That’s exactly why we published it.

But there’s a line where a spreadsheet stops being enough. When you’re juggling multiple entities, wondering why an 18% gross margin turned into a 4% net, staring down retainage and WIP reporting for bonding, or trying to figure out what this year’s decisions will cost you next April — that’s where a professional forecasting and KPI system earns its keep, and where our team spends most of its time with construction and contracting clients.

Second Mile Financial Services is a licensed Texas CPA firm in The Woodlands, and we work with contractors and trades businesses across the greater Houston area. If you build your 13-week forecast and it raises more questions than it answers, bring it to us. Walking in with clear numbers and specific questions makes any advisor’s help faster, cheaper, and far more useful.

Asking for help early is a sign of a business run well. It’s never a weakness.

Frequently asked questions

How long does it take to set up a 13-week cash flow forecast?

About an hour for a rough first version, using Excel, Google Sheets, or even a paper ledger. Expect to spend a little more time in the first two or three weeks refining payment dates and catching expenses you forgot. After that, keeping it current takes about 15 minutes a week.

How accurate is a 13-week forecast?

The first month is usually quite reliable, the second is decent, and the third is an educated guess. That’s expected — the far weeks aren’t there for precision, they’re there for direction. Because you update the forecast weekly, every week gets more accurate as it approaches. The forecast doesn’t need to be perfect to warn you; it just needs to be honest.

Do I need special software, or is Excel enough?

Excel or Google Sheets is enough, and for most contractors under a few million in revenue it’s what I recommend starting with. Building the sheet by hand teaches you how cash actually moves through your business. Software earns its place later, when you have multiple entities or enough volume that manual entry becomes the bottleneck.

How much cash reserve should a contractor keep?

A practical target is three months of fixed expenses, or the cost of your most expensive single surprise — for many trades businesses, that’s a replacement truck. Trades with strong seasonality, like HVAC, should lean toward the higher end, because the slow months arrive on schedule whether you’ve planned for them or not.

What’s the difference between a cash flow forecast and a budget?

A budget says what you plan to earn and spend, usually by month, and it mostly looks at categories. A cash flow forecast says when money actually enters and leaves your bank account, week by week. A budget can look perfectly healthy while you miss payroll, because budgets ignore timing. The forecast exists precisely to catch timing.

Stop guessing. Start seeing.

Fourteen columns. Four questions per week. Fifteen minutes to maintain. That’s the entire distance between checking your bank app twice a day and knowing — actually knowing — where your cash stands for the next three months.

Build the first column this week. And if you’d rather have an experienced set of eyes on it, we’re right here in The Woodlands — (281) 826-0100.