Introducing the Balloon Float Calculator

I’m excited to announce the launch of our new interactive Balloon Float Calculator, a comprehensive educational tool for designing superpressure balloon flights. This calculator replaces our previous float calculator with a more detailed, step-by-step approach that helps users understand the physics behind balloon flight.

Recent Updates (November 14, 2025)

The calculator has been significantly enhanced with several major improvements:

  • Physics validation - All calculations now validated against the Stratosonde Python Predictor for accuracy
  • Mylar balloon focus - Updated for spherical mylar superpressure balloons with proper burst pressure specifications (8-15 kPa typical, 15-25 kPa heavy-duty aerospace)
  • Enhanced pressure visualization - Pressure chart now displays altitude on y-axis for more intuitive atmospheric profile (like standard meteorological soundings)
  • Clearer physics explanations - New intuition boxes explain the critical distinction between net force (goes to zero at float) vs pressure differential (maximum at float)
  • Three interactive charts - Added volume/ascent rate and force analysis visualizations alongside the pressure chart
  • Step-by-step calculations - Each physics step now shows the actual equations and values used

Try the Calculator Now →

Balloon Float Calculator Interface

Understanding Balloon Physics: The Swimming Pool Analogy

Before diving into the technical details, let’s build an intuitive understanding using a familiar experience: diving in a pool.

Imagine you’re diving with a weight belt pulling you down. At the surface, the weight overcomes your buoyancy and you sink. As you descend, water pressure increases and compresses the air in your lungs. Eventually, you reach a depth where you neither sink nor float—you’ve achieved neutral buoyancy, perfectly balanced between the weight pulling you down and the water pushing you up.

As you dive deeper, the water pressure continues to increase and compress your lungs further. At some point, your lungs reach their minimum compressed volume and can’t shrink anymore. If you were to dive even deeper beyond this point, the pressure difference between the water outside and the remaining air inside your lungs would continue to grow, creating increasing stress on your chest.

Here’s the crucial insight: Your high-altitude balloon works exactly the same way, just in reverse. Instead of diving into increasing water pressure, it rises through decreasing air pressure. Instead of sinking to find neutral buoyancy, it ascends to find it. The physics are identical.

But there’s one critical difference: as a diver, you can adjust your depth by swimming up or down to find your preferred level. Your balloon cannot. Every parameter—the balloon size, payload weight, gas volume, free lift—must be calculated perfectly before launch. Once released, physics alone determines where it will float. One shot to get it right—which is exactly what this calculator helps you do.

Design Parameters

The calculator takes your design decisions and predicts your flight performance. Here’s what you control and what gets calculated:

Inputs (Your Design Decisions)

Balloon Volume (e.g., 0.5 m³)

  • Physical capacity of your balloon when fully inflated
  • Larger volume = higher float altitude, but heavier and more expensive
  • Typical party balloons: 36” Qualatex = 0.5 m³

Balloon Weight (e.g., 105 grams)

  • Mass of the latex envelope itself
  • Heavier balloon needs more lift and reduces maximum altitude
  • Typical 36” Qualatex weighs ~105g

Payload Weight (e.g., 10 grams)

  • Total mass of electronics, sensors, battery, antenna
  • Every gram matters—lighter payload = higher float altitude
  • Picoballoon payloads typically 5-15 grams

Free Lift (e.g., 8 grams)

  • Extra buoyant force beyond neutral buoyancy
  • Determines ascent rate and ability to overcome thermals
  • Too little = slow rise; too much = wastes gas and lowers float altitude
  • Typical range: 5-10 grams for picoballoons

Gas Type (Helium vs Hydrogen)

  • Helium: Safer, 0.169 kg/m³ density
  • Hydrogen: More lift (~2×), 0.085 kg/m³ density, but flammable
  • Choice affects both lift capability and safety considerations

Launch Conditions (Elevation, Temperature, Pressure)

  • Atmospheric conditions at your launch site
  • Affects initial air density and gas expansion calculations
  • Critical for accurate fill volume determination

Outputs (Predicted Flight Performance)

Gas Fill Volume (e.g., 0.28 m³ or 280 liters)

  • Exact amount of helium/hydrogen to fill at launch
  • Too little = insufficient lift; too much = reduced float altitude
  • This is the number you’ll use when filling your balloon

Superpressure Onset Altitude (e.g., 8,500 m)

  • Height where balloon becomes fully inflated
  • Below this: balloon expands freely as external pressure decreases
  • Above this: pressure builds inside the envelope
  • Understanding this helps predict balloon behavior during ascent

Float Altitude (e.g., 10,200 m or 33,500 ft)

  • Final cruising altitude where neutral buoyancy is achieved
  • Where your balloon will spend most of its flight
  • Determines radio line-of-sight and available power (solar angle)

Differential Pressure (e.g., 3.2 kPa or 0.46 psi)

  • Stress on balloon envelope at float altitude
  • Critical safety parameter—too high risks burst
  • Qualatex 36” bursts at ~4.7-5.4 kPa
  • Must stay well below burst threshold for safety margin

The Core Trade-offs

The picoballoon design challenge involves balancing several competing factors:

Payload Weight: Lighter payload = higher float altitude. Every gram of payload weight requires additional lift and lowers your maximum achievable altitude.

Balloon Size: Larger balloon = higher float altitude BUT heavier envelope and higher cost. A bigger balloon provides more buoyancy but adds its own weight penalty.

Differential Pressure: Must stay well below burst limit (typically 4.7 kPa for Qualatex 36”). Higher float altitudes generally mean higher differential pressures, so you’re balancing altitude goals against envelope stress limits.

Free Lift: Needs to be sufficient to overcome thermals and ensure steady ascent, but excess free lift wastes gas and can reduce float altitude.

The calculator helps you find the optimal combination where all these factors align for a successful, safe flight.

Features

Step-by-Step Physics Explanations

Rather than just showing final numbers, the calculator walks through each stage:

  1. System Density - Overall density of your balloon+payload system
  2. Gas Fill Volume - Launch fill calculation based on atmospheric conditions
  3. Float & Onset Altitude - Where the balloon inflates fully and achieves neutral buoyancy
  4. Pressure Analysis - Safety calculations for differential pressure and envelope stress

Visual Feedback

  • Color-coded calculation steps for clarity
  • Interactive pressure vs altitude chart showing external pressure, internal pressure, and differential pressure
  • Real-time safety indicators with clear warnings

Safety First

The calculator provides clear safety guidance for mylar superpressure balloons:

  • ✅ Safe (< 6 kPa) - Conservative safety margin for typical mylar balloons
  • ⚠️ Caution (6-8 kPa) - Approaching burst limit, careful monitoring needed
  • 🛑 Danger (≥ 8 kPa) - At or above typical burst limit, redesign required

Mylar Balloon Specifications:

  • Typical mylar superpressure balloons: 8-15 kPa burst pressure (polyester film with metallic coating)
  • Heavy-duty aerospace mylar: 15-25 kPa burst pressure (multi-layer construction)
  • Sealing quality is critical - always test pressurize on ground before flight to check seam integrity

Physics Validation & Heritage

This calculator builds upon the excellent UKHAS SPLAT float1g calculator, which has been used by the high-altitude balloon community for years. All physics calculations have been validated against the Stratosonde Python Predictor to ensure accuracy.

We’ve enhanced the original SPLAT calculator with:

  • Modern, responsive interface
  • Step-by-step educational explanations with intuition boxes
  • Three interactive charts (pressure analysis, volume/ascent rate, force analysis)
  • Real-time safety feedback
  • Physics validation against full trajectory predictor
  • Detailed calculation breakdowns showing equations and values

Try It Out

The calculator is now live at: https://stratosonde.github.io/.github/profile/float1g_visual.html

Technical Details

The calculator is built with:

  • Pure JavaScript for fast, client-side calculations
  • Chart.js for pressure visualization
  • Responsive CSS design that works on mobile and desktop
  • Atmospheric model calculations using standard formulas (barometric formula, ideal gas law, etc.)

Acknowledgments

Thanks to the UKHAS community for their pioneering work on balloon flight calculators and for sharing their knowledge openly. This tool stands on the shoulders of their excellent documentation and community wisdom.

Full Trajectory Prediction

While this calculator provides the equilibrium state (where the balloon will float), for complete multi-day flight predictions including:

  • Diurnal altitude oscillations (±500-2000 m from thermal cycles)
  • Real-time wind field evolution
  • Time-dependent thermal dynamics
  • Complete trajectory tracking over days/weeks

Use the full Stratosonde Python Predictor for detailed trajectory simulation.

What’s Next

Potential future enhancements:

  • Export/save calculator configurations
  • Multiple balloon comparison mode
  • Integration with real-time atmospheric data

Happy ballooning, and stay safe!