Converting a bike you already love into an electric one is one of the most satisfying projects a DIY cyclist can take on. For a fraction of the price of a factory e-bike, you can keep the frame and geometry that already fit you, learn how the whole system works, and end up with a machine tuned exactly to your commute, your hills and your budget. This guide is the big-picture map: every major decision, what it costs, how the law shapes it, and where to dig deeper.
Why convert instead of buying a factory e-bike?
The headline reason is value. A solid conversion can cost a few hundred pounds or dollars, where a comparable purpose-built e-bike often runs into four figures. But cost is only part of the story. Many cyclists already own a bike with a frame, fork and contact points dialled in over years of riding, and a good factory e-bike rarely matches that fit. Converting lets you keep what works.
There is also the matter of repairability and understanding. When you build the system yourself, you know where every cable runs and how every connector mates. A controller fault or a flat tyre on a sealed proprietary e-bike can mean a dealer visit; on a conversion, you fix it on your own bench. The trade-off is that you take on the engineering decisions a manufacturer would otherwise make for you, which is exactly what this guide exists to help with.
Hub motor or mid-drive? The first big fork in the road
Almost every conversion uses one of two motor types, and the choice shapes everything else.
Hub motors
A hub motor is built into the centre of a wheel, usually the front or rear. It bolts in, drives the wheel directly, and never touches your chain or gears. Hub kits are the cheapest and simplest to fit, which makes them the natural choice for flat-to-rolling commutes and first-time builders. Geared hubs are lighter and freewheel nicely; gearless direct-drive hubs are heavier, near-silent, and the only type that can offer (minimal) regenerative braking.
Mid-drive motors
A mid-drive mounts at the bottom bracket and sends its power through your chain and cassette. Because it leverages your existing gears, it climbs steep hills far better and keeps weight low and central for balanced handling. The catch: it costs more, is fiddlier to install, and accelerates wear on your chain, cassette and chainring because all that extra power runs through the drivetrain.
If you only read one comparison before buying, make it our dedicated hub motor vs mid-drive conversion kit breakdown. For the practical bolt-by-bolt side of the most common path, see installing a hub motor conversion kit.
Sizing the battery
The battery is usually the single most expensive component and the part people get wrong most often. Capacity is measured in watt-hours (Wh), and the maths is simple: Wh = volts × amp-hours. A 48 V pack rated at 10 Ah holds 480 Wh.
To size a pack, estimate your energy use. Mixed-terrain riding with moderate assist typically lands somewhere around 7–20 Wh per kilometre, depending on rider weight, wind, hills and how hard you lean on the motor. Multiply by the distance you actually need on a single charge, then add a comfort margin so you never crawl home on an empty pack. Common conversion batteries fall in the 360–720 Wh range, which covers most commutes comfortably.
Voltage also has to match your kit; a 36 V controller wants a 36 V battery, a 48 V controller wants 48 V. Form factor matters too: downtube packs look tidy and keep weight central, rear-rack packs are easy to fit but raise the centre of gravity, and frame-bag packs suit bikes with no bottle mounts. Cell quality (18650 vs 21700) and a good battery management system (BMS) separate a safe, long-lived pack from a risky one. We go deep on all of this in the e-bike conversion battery guide, and translate watt-hours into real-world distance in e-bike conversion range explained.
Know the law before you buy
E-bike rules vary widely and a conversion makes it surprisingly easy to build something that is technically illegal on public roads. In broad strokes: the EU and UK use the EAPC framework of roughly 250 W continuous, pedal-assist only, with assistance cutting out around 25 km/h (15.5 mph). The US uses a three-class system, allowing up to 750 W and Class 3 pedal-assist up to 28 mph. Australia generally permits 250 W (or 500 W for certain pedelecs).
These are summaries, not legal advice, and the details (helmets, registration, where you may ride, throttle legality) change. Always check your local laws and the latest regulations before choosing a kit's power and speed. Our e-bike conversion laws guide walks through each framework and the traps a conversion can fall into.
What will it cost?
Budget hub kits often start around US$300 and cover the motor, controller and basic display. Quality mid-drive kits typically run US$700–1,200. Add a battery (frequently the biggest line item), a few tools, and small extras such as a torque arm, brake cut-off levers and connectors. As a rough guide a clean, reliable budget build might total a few hundred dollars, while a high-quality mid-drive build can approach the price of an entry-level factory e-bike. The full breakdown, including hidden costs and DIY-versus-shop install, lives in how much it costs to convert a bike to electric.
Which bikes suit conversion?
The best candidates are sturdy, well-maintained bikes with steel or quality alloy frames, plenty of frame clearance, and disc or strong rim brakes. Rigid hybrids and older mountain bikes are ideal first projects. Be cautious with very lightweight road frames, carbon frames, and bikes with unusual dropout spacing or thru-axles, which complicate motor fitment. Our best bikes to convert to electric guide covers what to look for and what to avoid.
Tools and the install in outline
Most conversions need only hand tools: Allen keys, spanners, a chain whip and cassette tool (for rear hubs), tyre levers, a torque wrench and zip ties. A typical hub install means swapping in the motor wheel, fitting a torque arm to stop the axle spinning out of the dropouts (essential on alloy frames), mounting the battery and controller, routing the wiring, fitting the pedal-assist sensor (cadence or torque) and any throttle or display, then setting up brake cut-off switches. A first build often takes an afternoon; a mid-drive takes longer.
Sensors, throttles and displays
How your conversion reads your effort and shows you information is easy to overlook but shapes the ride every day. The pedal-assist sensor (PAS) is the key piece. A cadence sensor simply detects that you are turning the pedals and applies a preset level of assist, which feels like an on/off boost and is cheap and common. A torque sensor measures how hard you are actually pushing and scales the motor's help to match, giving a far more natural, bike-like feel that many riders prefer once they have tried it. A throttle lets you draw power without pedalling at all, which is handy for pulling away but is restricted or banned under some legal frameworks, so check before you fit one. The display ties it together, showing speed, battery state, assist level and often letting you tweak settings. None of these need to be expensive, but choosing thoughtfully here is the difference between a bike that feels like a moped and one that feels like a stronger version of itself.
Common mistakes to avoid
A few errors trip up first-time converters again and again. Skipping the torque arm on an alloy-dropout frame is the most dangerous. Buying an oversized battery you will never need wastes money and adds dead weight, while buying one too small leaves you nursing a flat pack home. Choosing a kit more powerful than your local law allows turns a bicycle into an unregistered motor vehicle. Overlooking brake cut-off switches means the motor can keep driving as you try to stop. And rushing the wiring, with loose connectors and unsecured cables, causes most of the intermittent faults people later blame on the kit. Slow, careful, and legal beats fast and powerful every time.
Safety essentials
Three things keep conversions safe. First, torque arms: lithium batteries and motor torque both demand respect, and an alloy dropout can fail without one. Second, the battery: buy quality cells with a real BMS, charge on a hard surface away from exhausts and flammables, and never charge a damaged pack. Third, braking and tyres: more speed and weight mean you need brakes and rubber that can cope. Always test at low power first.
Next steps
Start by deciding hub versus mid-drive based on your hills, then size a battery to the range you genuinely ride, and confirm the legal limits where you live before you spend a penny. Use the converter and range tools on our home page to sanity-check speed, cadence and distance figures, then work through the deeper guides linked above in that order. Measure your bike's dropout spacing and wheel size, make your shortlist, and you will be ready to order a kit with confidence rather than guesswork.