See exactly how much working capital you could free up by switching from slow emails to instant WhatsApp payments.
Late payments trap over £23 billion in unpaid invoices across UK small businesses at any given time. This calculator shows you exactly what your bank balance looks like month by month — and what happens when you cut your average payment time from 45 days to 7.
Adjust your monthly income, expenses, and average debtor days to model a realistic 12-month cash flow forecast. Then see the cash flow impact of faster invoice payment side by side.
Model and compare:
The total value of invoices you send in an average month.
How many days does it usually take for cash to hit your bank after you send an email invoice? (UK Avg is 45).
Trapped in Invoices
£0
Money owed to you right now
Potential Cash Unlocked
£18,000
Based on a 60% faster payment cycle (industry average for WhatsApp).
See how PaidInstantly shrinks your waiting time from 45 days down to 18 days.
The average open rate for business emails is 21.5%, with an average
response
time of 90 minutes.
(Source: Mailchimp Benchmarks
2025)
WhatsApp messages have a 98% open rate, with 90% of messages read within 3
minutes.
(Source: Gartner / Twilio Market
Research)
Disclaimer: This calculator provides an estimation based on industry average improvements in communication speed. While WhatsApp drastically improves invoice visibility, actual payment times depend on your clients' liquidity and payment terms. PaidInstantly improves the speed of delivery, but cannot force a client to pay if they do not have the funds.
Input your monthly turnover. We calculate the daily cost of waiting for funds.
Email invoices have a low 20% open rate. They get buried, forcing you to chase.
WhatsApp has a 98% open rate. We cut your payment time by ~60%, unlocking cash.
No credit card required • Syncs with Xero
Ask any UK accountant or bookkeeper what their clients' biggest operational problem is, and the answer is almost always the same: cash flow. Not profitability — cash flow. A business can be profitable on paper and still run out of money to pay its suppliers, staff, and HMRC if its invoices aren't collected promptly. The gap between issuing an invoice and receiving the cash is the single most dangerous number on any SME balance sheet.
Debtor days — formally known as Days Sales Outstanding (DSO) — measures the average number of days it takes your business to collect payment after a sale. The formula is simple: divide your accounts receivable by total revenue, then multiply by the number of days in the period. The UK SME average sits around 42–47 days, while the legal payment term specified in most contracts is 30 days. That gap of 12–17 extra days per invoice represents a significant, hidden cost to your business.
On a £20,000 monthly revenue business, those extra 15 days represent approximately £10,000 of cash permanently "on loan" to your clients at 0% interest. At scale, this becomes existential. It's why businesses with large order books still collapse — they're profitable, but illiquid.
The traditional invoice workflow — create in Xero, email as PDF, wait — is fundamentally broken. Business email has an average open rate of just 21%, meaning nearly 4 in 5 invoices you send via email are never even seen on the day they arrive. They land in crowded inboxes, get buried by marketing emails, or sit unread until the client's next "inbox zero" session. The result is that even clients who want to pay promptly simply don't see your invoice for days or weeks.
Compare this to WhatsApp, which has a 98% open rate with 90% of messages read within 3 minutes delivery. Studies by Twilio and Salesforce consistently shows that WhatsApp messaging reduces average payment collection times by 55–65% compared to email for equivalent invoices.
Reducing your debtor days from 45 to 18 doesn't just mean "you get paid sooner." It creates a compounding operational benefit: you can take on more work without needing a business overdraft, you can pay suppliers earlier to access early-payment discounts, you reduce the risk of bad debts (clients who see invoices quickly are less likely to dispute them), and you eliminate the time your team spends chasing payments manually. PaidInstantly clients typically see a reduction of 8–12 hours per month per employee on accounts receivable admin alone.
1. Calculating "Trapped Cash"
To find how much money is currently stuck on your balance sheet, we first calculate your daily revenue rate, then multiply by your average payment delay.
2. The "Unlocked Cash" Injection
Accelerating payments creates a one-time cash injection. We calculate this by identifying the gap in "Days Paid" and multiplying it by your daily revenue.
*Note: This is a simulation based on the assumption that WhatsApp invoicing reduces debtor days by 60%, a figure derived from industry benchmarks for messaging vs. email open rates.
The 60% figure is based on aggregate data from Twilio's State of Customer Engagement report and Salesforce's messaging benchmarks, comparing email vs. WhatsApp payment collection for equivalent SME invoice volumes. Individual results vary depending on your industry, invoice size, client base, and existing payment terms. Businesses in professional services and construction typically see the largest improvements; retail with established card payment infrastructure may see smaller but still significant gains.
Trapped cash is the total value of invoices that have been issued but not yet paid at any given moment. If your monthly revenue is £20,000 and your debtor days are 45, then at any point in time you have approximately £30,000 of money you've already earned but haven't received. This is the cash that could be in your bank account — earning interest, paying suppliers, or funding growth — but is instead sitting as an asset on your balance sheet with no guarantee of collection.
The simulator models the cash flow improvement available from any significant reduction in debtor days — WhatsApp is simply the most impactful lever most SMEs haven't pulled yet. If you have any method of materially reducing the time between invoice issuance and client action (for example, switching to direct debit via GoCardless, or using payment links instead of bank transfer details), the principle is identical. The 60% improvement figure is specifically calibrated to the WhatsApp channel based on industry data.
PaidInstantly cuts average debtor days from 45 to 7 with automated WhatsApp reminders. UK bookkeepers and business owners use it to stop chasing invoices manually — and watch their cash flow improve without changing anything else.
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