How often should I bleed my brakes?

Bleed annually or when the lever feels spongy/inconsistent. DOT fluid absorbs moisture over time and should be replaced every 12 months. Mineral oil is more stable but still benefits from annual bleeds.

Can I mix DOT and mineral oil?

Absolutely not. DOT fluid in mineral oil brakes (or vice versa) will destroy the seals and caliper. Always check your brake system type before purchasing fluid.

How much fluid do I need to buy?

A 100 ml bottle is usually enough for 2-3 full sets (front + rear) — a single full set typically consumes around 50 ml. This calculator gives a generous per-brake purge allowance (roughly 3x the system volume) so you always have enough on hand; the figure is the amount to buy, not the fluid the brake actually holds.

Can I bleed brakes without a proper kit?

It is possible with DIY syringes, but a proper bleed kit for your specific brakes makes the job much easier and reduces the risk of introducing air. Kits cost £15-30.

Why are my brakes spongy after a bleed?

Air is still in the system. Re-bleed, making sure to tap the hose and calliper to dislodge trapped bubbles. Ensure the calliper is the lowest point when bleeding Shimano brakes.

BikeCalcs

Bleed Volume Calculator — Hydraulic Brake Fluid

Calculate the correct hydraulic brake fluid volume for bleeding your brakes.

Brake SystemYour brake brand/family — determines fluid type (mineral oil vs DOT 5.1) and approximate system volume. Never mix the two.

Hose LengthFront / small-frame hoses are shorter and hold less fluid; rear / large-frame hoses are longer and need a little more.

How We Calculate This

Bleed Volume Calculation

System volume: 15-25 ml per brake depending on system
Hose length adjustment: Short (-15%), Standard (baseline), Long (+20%)
Total fluid needed = System volume × 3 (a generous purge allowance to fully flush old fluid, not the amount the brake actually holds)
In practice a full front + rear set uses about 50 ml, so a 100 ml bottle covers 2-3 full sets — the per-brake figure here is deliberately conservative so you never run short mid-bleed
Different brake systems use different fluids — always check before buying, and never mix mineral oil with DOT

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.