Speedo Biofuse Fitness Fin Size Guide

The Speedo Biofuse Fitness Fin is a short, dual intensity fin with stiff blades designed to create maximum resistance in the water. Suitable for swimmers of all levels, the Biofuse Fitness Fin is ideal for fitness training, technique work, and building leg strength and endurance. Available in sizes XS to XL, covering UK shoe sizes 12 Junior to Adult 13.

Select your size using your UK shoe size in the table below. Where your shoe size falls at the overlap between two sizes, we recommend sizing up for a more comfortable fit.

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💪 Strengthen Leg Muscles Added resistance increases the workload on your leg muscles, building strength and endurance with every session.
⚡ Increase Workout Speed Extra propulsion adds speed with minimum effort, leaving you free to focus on and ingrain correct technique.
🦶 Dual Intensity & Short Blade Stiff blades create maximum resistance. The short blade design is ideal for fitness training and technique drills.
🔄 Biofuse Comfort Flexible foot pocket construction adapts to your foot shape for a secure and comfortable fit during extended training sets.
Sizing Table — Speedo Biofuse Fitness Fin
Size UK Shoe Size EU Shoe Size US Shoe Size
XS 12–1 Jnr 31–33 1–3
S 2–4 34–37 3–5
M 5–7 38–41 6–8
L 8–10 42–45 9–11
XL 11–13 46–48 12–14

Sizing is based on UK shoe size. Where your shoe size falls at the boundary between two sizes, we recommend sizing up for a more comfortable fit. The open-toe foot pocket design accommodates a range of foot widths within each size band.

Fit & Usage Notes — Speedo Biofuse Fitness Fin
  • Select your size using your UK shoe size. If you are between sizes, size up for a more comfortable fit — fins that are too tight can cause discomfort and cramping during longer training sets.
  • The Biofuse foot pocket is designed to flex and adapt to the shape of your foot. The fit will feel firmer when dry and ease slightly once wet — this is normal.
  • The open-toe design accommodates a range of foot widths within each size band, making the Biofuse Fitness Fin suitable for wider feet that may find enclosed pocket fins restrictive.
  • The stiff blades are designed to create maximum resistance. If you are new to fin training, start with shorter sets and build up duration gradually to allow your legs to adapt to the increased workload.
  • Biofuse Fitness Fins are suitable for use in all four strokes and are particularly effective for front crawl and backstroke kick sets, as well as drills focusing on body position and rotation.
  • Always rinse in fresh water after every use and allow to dry naturally away from direct sunlight. Do not leave fins in direct sun for extended periods as this can degrade the rubber compound over time.
  • Unsure? Contact our team or call +44 1827 330600. Speak to a member of staff at Allens if you are unsure.
Frequently Asked Questions — Speedo Biofuse Fitness Fin

Select your Biofuse Fitness Fin size using your UK shoe size and the sizing table above. The fins are available in XS to XL, covering UK shoe sizes 12 Junior to Adult 13. If your shoe size falls at the boundary between two sizes, we recommend sizing up for a more comfortable fit. Speak to a member of staff at Allens if you are unsure.

A dual intensity fin has a blade stiffness that creates two levels of resistance as you kick — a higher resistance on the downbeat and a lower resistance on the upbeat. The Speedo Biofuse Fitness Fin uses stiff blades specifically designed to create maximum resistance in the water, making it effective for building leg strength and cardiovascular fitness in a pool training environment.

Yes. The Biofuse Fitness Fin is suitable for swimmers of all levels. If you are new to fin training, introduce them gradually — start with shorter sets (e.g. 100–200m) and build up over time as your legs adapt to the additional resistance. The short blade design and Biofuse foot pocket make them straightforward to use and comfortable for extended wear once broken in.

Fitness fins (like the Speedo Biofuse Fitness Fin) have short, stiff blades designed to create maximum resistance, prioritising strength and cardiovascular development. Training fins typically have longer, more flexible blades that focus on technique development and body position by increasing speed through the water. The right choice depends on your training goals — resistance and strength building vs. technique and speed development.

Biofuse Fitness Fins are most commonly used for front crawl and backstroke kick sets and drills. They can be used for butterfly kick (dolphin kick) sets, but the stiff blade may feel more restrictive than a longer, more flexible monofin or training fin for butterfly-specific work. They are not recommended for breaststroke as the outward kick motion is not suited to standard fin design.

Fins that are persistently too tight can cause blisters and cramping during training. Speak to a member of staff at Allens if you are unsure which size to choose.

Biofuse Fitness Fins are designed for pool use. While they can be used in open water, the short stiff blade is optimised for pool training drills rather than open water propulsion. Always rinse thoroughly in fresh water after any use in the sea or open water to remove salt and debris, as these can degrade the rubber compound over time.

Rinse your Biofuse Fitness Fins in cold fresh water after every use to remove chlorine, salt, and debris. Allow them to dry naturally away from direct sunlight and heat sources — prolonged UV exposure and high temperatures can degrade the rubber compound and reduce the lifespan of the fins. Store them flat or hanging in a cool, dry place. Do not fold or crush the blades during storage as this can cause permanent deformation.