How to Build a 10-Year Financial Forecast (And Why Most People Never Do)

Why financial forecasting matters more than budgeting

Most personal finance advice focuses on budgeting — tracking what you spend this month. That's useful, but it answers the wrong question. The question that actually determines your financial future isn't "did I overspend on groceries?" It's: "What does my financial picture look like in 5, 10, or 20 years?"

A financial forecast answers that question with real numbers. And once you see the numbers, the decisions become obvious.

The four ingredients of a financial forecast

A meaningful long-term financial forecast needs four inputs:

1. Current balances — what you have today (savings, investments, pension, debt)
2. Income streams — salary, freelance income, rental income, dividends
3. Expenses — fixed costs, variable spending, loan repayments
4. Growth rates — interest on savings, investment returns, inflation, pension contributions

Most spreadsheet forecasts fail because they miss the interaction between these four inputs. A savings account doesn't just sit there — it compounds. Expenses don't stay flat — they inflate. Investments don't return a flat percentage — they have management fees that compound against you.

A proper forecast models all of this together, month by month.

Step 1: List every financial item with its current balance

Start with a complete inventory. Don't skip anything because it feels small. A €5,000 emergency fund today, at 4% interest, becomes €7,400 in 10 years. List:

• Bank accounts and cash savings
• Investment accounts (stocks, ETFs, index funds)
• Pension or retirement accounts
• Any money you owe (loans, credit cards, mortgages)
• Assets you plan to liquidate (a car you'll sell, a bond that matures)

Step 2: Add every recurring income and expense

This is where forecasts get detailed. List:

• Monthly salary (after tax)
• Any other regular income (side income, rental yield, investment dividends)
• Fixed monthly expenses (rent, mortgage, subscriptions, insurance)
• Variable monthly expenses (groceries, transport, entertainment) — use realistic averages
• Annual or irregular expenses (car insurance, holidays, annual fees) — divide by 12 to spread them

The difference between total income and total expenses is your monthly surplus (or deficit). This surplus flows into your savings and investments each month.

Step 3: Apply growth rates and fees

This is what separates a real forecast from a simple spreadsheet. For each asset:

• Savings account: apply the annual interest rate, compounded monthly
• Investment portfolio: apply annual return assumption (e.g., 7% for a diversified index fund), minus annual management fee (e.g., 0.15% for a low-cost ETF, 1–2% for active funds)
• Pension: add both employee and employer contributions, apply growth rate, apply management fee
• Expenses: apply inflation (typically 2–3% annually in developed economies, higher in some regions)

The management fee effect surprises most people. A 1.5% annual fee on €100,000 over 20 years doesn't cost €30,000 — it costs over €90,000, because the fee compounds against your growth every year.

Step 4: Model the scenarios that matter to you

A single forecast is useful. Multiple scenario forecasts are transformative. Common scenarios to model:

• Baseline: current trajectory, no changes
• Save more: what if you increased savings by €300/month?
• Career change: what if income dropped 20% for two years then grew faster?
• Buy a home: what if you spent €50,000 on a down payment and added a mortgage?
• Early retirement: what if you stopped working at 55? Does the money last?

When you compare these scenarios side-by-side, the right decision often becomes obvious — or at least, far better informed.

The compounding effect over 10 years

Here's an example that shows why a 10-year forecast changes how you think:

Two people both earn €50,000/year and spend €40,000. Person A puts the €10,000 surplus in a savings account at 3%. Person B invests it in an index fund at 7% with 0.15% fee.

After 10 years: Person A has approximately €116,000. Person B has approximately €145,000. The €29,000 difference comes purely from investment return — with no extra saving required.

Over 20 years, the gap widens to over €140,000. This is the power of seeing your forecast before you make the decision.

Building your forecast in MyRunway

MyRunway's financial forecasting tool does all of this automatically. You enter your financial items once — income, expenses, savings, investments — and it builds a month-by-month forecast up to 30 years forward. It handles compound interest, management fees, inflation, and scenario comparison natively.

The free plan gives you a 3-year forecast with up to 2 scenarios. The Premium plan extends this to 15 years, and Pro to 30 years with scenario sharing for couples or collaborators.

The right time to start a financial forecast is before you make your next big financial decision — not after.