You have been quoted £120,000 for a new CNC machine that your production manager says will transform turnaround times. The sales pitch includes a cash flow forecast showing £35,000 a year in savings and extra output for five years. On paper it looks brilliant. But before you sign the purchase order, there is one number missing from every supplier’s pitch: the discount rate you should use to test whether those future cash flows are actually worth more than the £120,000 you are being asked to pay today. Get that number wrong and you could approve a loss-making project, or reject a genuinely profitable one. That number is your Weighted Average Cost of Capital, or WACC.
What WACC Actually Means for Your Business
Every pound your business uses to fund a new machine, a new hire, or a new premises comes from somewhere, and none of it is free. If you borrow from the bank, you pay interest. If you use money that shareholders or owners have put into the business — or profits you have chosen to keep in the business rather than pay out — those owners still expect a return, even if no invoice ever arrives for it. WACC blends these two costs together, weighted by how much of each type of funding your business actually uses, to give you one single percentage: the minimum return any investment needs to generate just to break even on the cost of the money behind it. Anything below that rate destroys value, even if the project looks profitable on a simple cash-in, cash-out basis.
The Three Ingredients: Cost of Debt, Cost of Equity, and Weights
Cost of debt is usually the easiest piece. If your bank charges 6% on a loan, that is your starting point — but in the UK, loan interest is tax-deductible, so the real cost to the business is lower once you account for corporation tax relief. Cost of equity is trickier because there is no invoice for it; it represents the return your owners or shareholders expect for the risk of having their money tied up in the business rather than somewhere safer. Many SME owners use a figure based on what they could earn elsewhere for similar risk, often somewhere between 10% and 15%. Weights reflect the proportion of capital from each source — in corporate finance theory, market values are preferred, though private SMEs typically use balance sheet book values or target capital structures as practical proxies. More debt relative to equity pulls your WACC down because debt is normally cheaper, but it also increases financial risk.
Worked Example: Should Harrogate Engineering Buy the £120,000 Machine?
Harrogate Engineering Ltd is a manufacturing SME funded by £300,000 of owner’s capital and a £200,000 bank loan charged at 6% interest. The owner wants a 12% return on the capital tied up in the business, reflecting the risk of running a manufacturing SME rather than leaving the money in a savings account. Corporation tax is 25%. The finance director needs to work out the discount rate to test a proposed £120,000 CNC machine, expected to generate £35,000 of extra net cash flow every year for five years.
| Source of capital | Amount | Weight | Cost | Weighted cost |
|---|---|---|---|---|
| Owner’s equity | £300,000 | 60% | 12% required return | 7.2% |
| Bank loan | £200,000 | 40% | 6% interest, 4.5% after 25% tax relief | 1.8% |
| Total capital employed | £500,000 | 100% | — | WACC = 9.0% |
| Cost of equity weighted (60% × 12%) | 7.2% |
| Cost of debt weighted after tax (40% × 4.5%) | 1.8% |
| Weighted Average Cost of Capital (WACC) | 9.0% |
Turning the Discount Rate into a Decision
With a WACC of 9%, the finance director discounts each year’s £35,000 cash flow back to today’s value, because £35,000 received in five years is worth less than £35,000 sitting in the bank today. The further into the future a pound arrives, the more it shrinks once discounted.
| Year | Net cash flow | Discount factor at 9% | Present value |
|---|---|---|---|
| 1 | £35,000 | 0.917 | £32,110 |
| 2 | £35,000 | 0.842 | £29,459 |
| 3 | £35,000 | 0.772 | £27,026 |
| 4 | £35,000 | 0.708 | £24,795 |
| 5 | £35,000 | 0.650 | £22,748 |
| Total | £175,000 | 3.889 | £136,138 |
| Present value of cash flows (Years 1 to 5) | £136,138 |
| Initial investment | £120,000 |
| Net present value (NPV) | £16,138 |
At a true WACC of 9%, this machine creates value for the business — £16,138 of it. But if Harrogate Engineering had used a cautious gut-feel discount rate of 15% instead, the same project would show a negative NPV of around minus £2,673 (or minus £2,675 using unrounded discount factors), and the owner would have rejected a genuinely profitable investment. The discount rate is not a formality on a spreadsheet; it is the hinge the whole decision swings on.
A very common mistake among SME owners is to use the bank loan’s interest rate as the discount rate for every investment decision, simply because it is the only financing cost they see on paper. This ignores the fact that equity capital is not free — owners and shareholders expect a return too, usually a much higher one than the bank charges. Using 6% instead of a properly blended 9% WACC would make almost any project look attractive, encouraging over-investment and quietly eroding the return owners actually earn on their own money over time.
| Account | Dr | Cr |
|---|---|---|
| Plant and machinery (fixed asset) | £120,000 | |
| Bank | £120,000 |
Recording the purchase of the CNC machine following a positive NPV assessment at the company’s 9% WACC, with payment made directly from the business bank account.
Why the Discount Rate Deserves More Attention Than the Spreadsheet Formulas
Most of the effort in investment appraisal goes into forecasting the cash flows — unit sales, cost savings, headcount reductions — and very little goes into questioning the discount rate applied to them. Yet as the Harrogate Engineering example shows, a few percentage points either way can completely reverse the decision. It is worth testing your project at your actual WACC, and then again at a rate one or two points higher and lower, just to see how sensitive the outcome is. It is also worth recalculating your WACC whenever your funding mix shifts meaningfully, because a discount rate calculated three years ago on a different balance sheet may no longer reflect the real cost of capital you are putting at risk today.
Takeaways You Can Apply This Week
- Never discount SME investment cash flows using your bank loan’s interest rate alone — it ignores the real cost of the equity you and other owners have tied up in the business.
- Work out your capital weights from your balance sheet or target funding mix: total equity divided by total capital, and total debt divided by total capital, then apply each cost accordingly.
- Always use the after-tax cost of debt in your WACC calculation, not the raw interest rate, since UK corporation tax relief genuinely lowers the cost of borrowing.
- Recalculate your WACC whenever your funding mix changes significantly — a new loan, an equity injection, or a change in dividend policy can all shift the rate you should be using for future decisions.
Not Sure Your Discount Rate Is Right?
Our team helps SME owners and finance managers build reliable WACC calculations, NPV models, and investment appraisals they can defend to the board, the bank, and themselves.
