Allocation Waterfall

An allocation waterfall is the fixed order in which a repayment is applied against what a borrower owes. Money coming in fills the first category until it is satisfied, then spills into the next, and the next, until the payment is used up. The standard order is penalties, then fees, then interest, then principal.

It exists because a repayment is almost never one thing. A borrower paying 1,200 against an instalment of 1,000 with a 150 late penalty and 45 accrued interest is not making a "principal payment" — the payment has to be split, and the split has to follow a rule that is the same every time.

Without a fixed rule, the same 1,200 can be recorded four different ways by four different loan officers, and none of them are wrong.

The standard waterfall

Payment received
      │
      ▼
1. Penalties      ── late fees, default interest, arrears charges
      │ (remainder)
      ▼
2. Fees           ── processing, insurance, legal, admin, recovery costs
      │ (remainder)
      ▼
3. Interest       ── accrued interest outstanding
      │ (remainder)
      ▼
4. Principal      ── the capital actually lent
      │ (remainder)
      ▼
5. Overpayment    ── credit balance, held against future instalments

Worked example. A borrower owes 150 in penalties, 0 in fees, 420 in accrued interest, and 8,000 in principal. They pay 1,000.

| Step      |   Due | Allocated | Payment left |
| --------- | ----: | --------: | -----------: |
| Penalties |   150 |       150 |          850 |
| Fees      |     0 |         0 |          850 |
| Interest  |   420 |       420 |          430 |
| Principal | 8,000 |       430 |            0 |

The loan's principal drops to 7,570. Penalties and interest are cleared. Nothing is left over.

Now the same borrower pays 400 instead:

| Step      |   Due | Allocated | Payment left |
| --------- | ----: | --------: | -----------: |
| Penalties |   150 |       150 |          250 |
| Fees      |     0 |         0 |          250 |
| Interest  |   420 |       250 |            0 |
| Principal | 8,000 |         0 |            0 |

Principal does not move at all. The loan is 8,000 outstanding with 170 of interest still accrued and unpaid — and, importantly, the instalment has not been satisfied, so the loan is still in arrears and still ageing. A lender applying that 400 straight to principal would have shown the borrower as making progress when they were falling further behind.

Why the order is penalties first

The sequence is not arbitrary. It runs from the most volatile, most recently incurred charge to the most stable.

Penalties first because they are the charge that keeps growing while unpaid. Leaving a penalty unsettled while reducing principal means the penalty carries into the next period and accrues again.

Fees next because they are typically one-off amounts already earned by the lender — recovery costs, legal costs, insurance premiums paid on the borrower's behalf. They are money you have already spent.

Interest before principal because interest is the price of the capital being outstanding. If a payment reduced principal while interest went unpaid, the unpaid interest would sit on the account indefinitely and, in most books, quietly capitalise. The loan's economics stop matching its schedule.

Principal last because it is the only component that does not grow. It is safe at the bottom of the waterfall.

Overpayment last of all. Anything left after principal should sit as a credit balance against the next instalment, not disappear into a suspense account nobody reconciles.

Where lenders vary

Some books put fees ahead of penalties. Some separate accrued interest from interest already billed and satisfy the billed portion first. Regulated lenders in some markets are required to apply payments principal-first on certain consumer products, or to apply them to the oldest instalment before the newest.

Any of these is defensible. What is not defensible is having no written rule, or having one that differs by branch. Pick an order, document it, configure it once, and apply it to every payment on every loan.

Instalment-level vs. account-level waterfalls

There is a second question the waterfall has to answer: which instalment does the payment target?

Oldest-first (account-level). The payment clears the oldest outstanding instalment completely — its penalties, fees, interest, principal — before touching the next one. This is the common approach and the one most borrowers expect. It clears arrears in the order they arose.

Current-first. The payment satisfies the current instalment before older arrears. Used occasionally in restructuring, where the goal is to get the borrower current going forward and deal with historic arrears separately.

Proportional. The payment spreads across all outstanding instalments. Rare, and difficult to explain to a borrower.

Oldest-first is the sensible default. It is the only one where "you are two instalments behind" stays true and understandable.

What goes wrong without a consistent waterfall

Your portfolio at risk is wrong. A partial payment applied to principal instead of the outstanding instalment can make a delinquent loan appear current. The loan stops ageing, drops out of PAR, and reappears three months later much worse. PAR is only as reliable as the allocation rule underneath it.

Your income is misstated. Interest and penalties are income. Principal is a balance sheet movement. Applying a payment to the wrong bucket moves money between your profit and loss and your balance sheet — which means your revenue figure, your tax position, and your provisioning are all slightly off, and the error compounds across thousands of payments.

Borrowers dispute their balances. If a borrower's statement cannot be reconstructed from their payment history, you will lose the argument — and in a regulated market, you may lose it formally. A documented waterfall makes every balance explainable line by line.

Interest keeps accruing on a loan you thought was shrinking. The most expensive version: principal was reduced on paper while unpaid interest accumulated in the background. The borrower believes they are nearly done. The system says otherwise. Someone has to have that conversation.

Two branches report different numbers for the same product. When allocation is manual, it drifts. One branch's officer clears penalties first, another's clears the instalment proportionally. Neither knows the other is different until an auditor lines the books up side by side.

The accounting entries

Each step of the waterfall hits a different account, which is why the order has to be right before the double-entry postings are generated, not after.

| Waterfall step | Debit       | Credit                               |
| -------------- | ----------- | ------------------------------------ |
| Penalties      | Cash / Bank | Penalty income                       |
| Fees           | Cash / Bank | Fee income                           |
| Interest       | Cash / Bank | Interest income                      |
| Principal      | Cash / Bank | Loans receivable                     |
| Overpayment    | Cash / Bank | Borrower credit balance (liability)  |

Get the allocation wrong and the entries are wrong. Recording the whole payment as a single credit to loans receivable — which is what most spreadsheet books effectively do — understates income and overstates principal recovery simultaneously.