How Convertible Notes Work: Complete Guide with Calculator

June 5, 2026 • 12 min read

Convertible notes are one of the most common instruments for early-stage startup fundraising. They're quick, flexible, and delay valuation discussions until later. But their mechanics—interest, caps, discounts, and conversion triggers—can confuse even experienced founders.

This guide explains exactly how convertible notes work, walks through conversion calculations, and includes a free interactive calculator to model your own scenarios.

What Is a Convertible Note?

A convertible note is a debt instrument that converts into equity at a future financing round. Think of it as a loan with an unusual repayment plan: instead of getting cash back, the investor receives shares.

Key characteristics:

• It's debt initially: The note has a principal amount, interest rate, and maturity date
• It converts later: At a qualified financing round (typically Series A), the note converts into equity
• It has investor protections: Caps, discounts, and interest ensure investors get a fair deal

Why Use Convertible Notes?

Key Terms Explained

Principal Amount

The cash amount invested. If an investor puts in $100,000, that's the principal. Interest accrues on this amount, increasing the total that converts.

Interest Rate

Convertible notes accrue interest (typically 4-8% annually), but this isn't paid in cash. Instead, it increases the principal amount that converts into equity.

Valuation Cap

The cap is the maximum price per share the investor will pay. It protects the investor by ensuring they get a minimum percentage regardless of how high the valuation goes.

Discount

The discount (typically 15-25%) gives the investor a cheaper price per share than the new investors pay.

Cap vs. Discount: Which Applies?

When both a cap and discount exist, the investor gets the lower conversion price. The conversion price is the lesser of:

• Cap-based price
• Discounted price

Maturity Date

The date by which the note must convert or be repaid. Typically 18-24 months. If no qualified financing occurs by maturity, the note converts at a preset price or must be repaid (though repayment is rare).

Conversion Mechanics: How It Actually Works

At a qualified financing round (usually $1M+), the note converts. Here's the step-by-step process:

1. Calculate the conversion principal: Original amount + accrued interest
2. Determine conversion price: The lower of cap-based or discount-based price
3. Calculate shares issued: Conversion principal ÷ Conversion price
4. Issue shares to note holder: These are added to the pre-money capitalization

The Conversion Formula

Complete Example

Multiple Notes: The Stacking Problem

When you have multiple convertible notes with different caps and discounts, conversion gets complex. The key principles:

• Each note converts separately: Calculate conversion for each note individually based on its own terms
• Order matters for equity: Earlier investors typically get better terms (lower caps)
• Total dilution adds up: All note holders receive shares before the Series A investment

Convertible Notes vs. SAFE

SAFEs (Simple Agreement for Future Equity) are similar but key differences exist:

• Debt vs. Equity: Notes are debt, SAFEs are not (no interest, no maturity)
• Maturity: Notes have maturity dates; SAFEs don't expire
• Complexity: SAFEs are simpler; notes have more terms
• Legal precedent: Notes have established case law; SAFEs are newer

Negotiation Tips for Founders

Valuation Cap

• Lower cap = more dilution: A $4M cap gives investors more shares than an $8M cap
• Set realistic caps: Too low and you dilute excessively; too high and investors won't bite
• Cap tiers: Consider different caps for different investors based on ticket size and timing

Discount

• Standard is 15-25%: Anything higher is aggressive; anything lower limits investor interest
• Discount vs. cap: The cap usually matters more in high-valuation scenarios
• Pro-rata rights: Large note holders often ask for pro-rata rights in the next round

Interest Rate

• 4-8% is standard: Higher rates increase dilution but are rarely the deciding factor
• Negotiable but minor: Most variation happens in caps, not interest rates

Model Your Conversion

Convertible note math gets complex fast. Use our free calculator to model your scenario:

Key Takeaways

• Convertible notes are debt that converts to equity at a future financing round
• Caps protect investors from overpaying; discounts reward early risk
• Interest increases the conversion amount but isn't usually paid in cash
• Conversion price is the lower of cap-based or discount-based price
• Multiple notes with different terms require individual calculations
• Model your conversion before signing to understand dilution impact