How much to set aside for small unexpected costs short of a full emergency.

An emergency fund covers losing your income for months. A rainy day fund covers the 400 boiler repair you didn't plan for this Tuesday. The two serve different purposes, and mixing them often means the emergency fund gets gradually eroded by small shocks until it's not really an emergency fund anymore.

How to use it

Enter your monthly discretionary income — income minus essential fixed costs (rent/mortgage, utilities, basic food, transport). The discretionary pot is what you'd divert to fix an unexpected cost. Multiply by the buffer month count you want (1-3 is typical).

What the result means

The primary figure is the target rainy day fund. The buffer serves two purposes: it absorbs small shocks without any planning, and it gives psychological cover so one-off overspends don't trigger a wider budget review.

Why it's separate from the emergency fund

Emergency funds exist to protect income loss — typically 3-6 months of essential expenses sitting in cash. Rainy day funds protect the emergency fund from being drained by small predictable-in-aggregate shocks. Keep them in different accounts if possible so the labels stay clean.

Run it with sensible defaults

Using monthly discretionary income of 1,500, buffer months of 2, the calculation works out to 3,000.00. Nudge the inputs toward your own situation and the output recalculates instantly. The defaults are meant as a starting point, not a recommendation.

The levers in this calculation

The inputs — Monthly Discretionary Income and Buffer Months — do not pull with equal force. Not every input has equal weight. Flip one at a time toward extreme values to feel which ones move the needle most for your situation.

How the math works

Straightforward multiplication: monthly discretionary income times the chosen buffer. The tool does not prescribe a specific month count — 1 month is a light buffer for steady income; 3 months is conservative for variable income. Keep the fund in instant-access savings, not a locked product, because the entire point is immediate availability. The working is transparent — you can verify every step yourself in the formula section below. No black box, no opaque "proprietary model".

Why the number matters

Saving without a target is like driving without a destination — you'll make progress, but you won't know when you've arrived. This tool gives you a concrete figure to work toward, which is the first step in turning a vague intention into an actual plan.

What this doesn't capture

The calculation assumes a steady savings rate and a stable interest rate. Real saving journeys include emergencies, windfalls, and rate changes — especially in easy-access products. The figure is a direction of travel, not a guarantee.