Battery Size Calculator

About the Battery Size Calculator

The Battery Size Calculator helps you select the right battery format for your project or device. With dozens of standard battery sizes available — from tiny coin cells to large D-cells and cylindrical 18650s — choosing the right one requires balancing capacity, voltage, physical dimensions, weight, and cost.

It gives a comprehensive comparison of all common battery sizes including AA, AAA, C, D, 9V, CR2032, 18650, 21700, and many more. Enter your requirements for capacity, voltage, and physical size constraints, and the calculator recommends the best battery formats for your application.

Whether you're designing a portable product, building a battery pack, or simply choosing replacement batteries, understanding battery sizes and their trade-offs is essential. The calculator includes detailed specification tables, physical dimension comparisons, and energy density rankings to help you make an informed choice.

Use the preset examples to load common values quickly, or type in custom inputs to compare different battery formats against your constraints.

When This Page Helps

Use this calculator when you need to match a device or pack design to the battery format that actually fits. It is useful for comparing capacity, size, and voltage tradeoffs before you commit to a cell type. That saves time when the enclosure or holder space is already fixed and narrows the choice to formats you can actually source.

How to Use the Inputs

1. Enter your minimum required capacity in mAh
2. Select the required nominal voltage for your circuit
3. Enter maximum physical dimension constraints (length, diameter, or width)
4. Optionally set a maximum weight limit in grams
5. Review the filtered list of compatible battery formats
6. Compare specifications in the detailed comparison table
7. Use the visual size comparison to see relative battery dimensions

Example Calculation

Tips & Best Practices

• For high-drain devices, check the battery's maximum continuous discharge rate, not just capacity
• 18650 and 21700 cells offer the best value per Wh for hobbyist and DIY projects
• Always verify that your charger is compatible with your battery's chemistry and size
• Coin cells like CR2032 are ideal for low-power applications like watches and key fobs
• Consider both upfront cost and long-term cost when choosing between disposable and rechargeable
• Store batteries in a cool, dry place — heat is the biggest enemy of battery longevity

Standard Battery Size Guide

Batteries come in standardized sizes defined by organizations like IEC and ANSI. The most common consumer sizes are AA (LR6), AAA (LR03), C (LR14), D (LR20), and 9V (6LR61). These designations vary by region and chemistry, but physical dimensions are standardized.

Cylindrical lithium-ion cells use a numeric naming convention: 18650 means 18mm diameter × 65mm length. Common sizes include 14500 (AA-sized Li-ion), 18650 (most popular), 21700 (newer, higher capacity), and 26650 (large format). These cells are the building blocks of laptop batteries, power tool packs, and electric vehicle battery systems.

Choosing the Right Battery Size

The selection process starts with your electrical requirements: voltage and capacity. Then consider physical constraints: available space, weight budget, and form factor. Finally, factor in practical considerations like cost, availability, cycle life, and whether you need rechargeable or disposable.

For consumer electronics, AA and AAA formats offer the widest availability and lowest cost. For hobbyist projects and high-performance applications, 18650 and 21700 lithium cells provide the best energy density. For compact devices, coin cells and small lithium polymer pouches offer the lowest profile.

Battery Pack Design Basics

When building battery packs, cells are combined in series (for higher voltage) and parallel (for higher capacity). The notation nSmP indicates n cells in series, m in parallel. For example, a 4S3P pack has 4 series groups of 3 parallel cells, giving 4× single cell voltage and 3× single cell capacity. Always use matched cells from the same batch and protect packs with a BMS (Battery Management System).

Frequently Asked Questions

• What do battery size numbers like 18650 mean?

  The first two digits are diameter in mm, the next two are length in mm, and a trailing 0 indicates cylindrical shape. So 18650 = 18mm diameter, 65mm long, cylindrical.

• What's the difference between rechargeable and disposable batteries?

  Disposable (primary) batteries like alkaline AA cells have higher initial capacity but can only be used once. Rechargeable (secondary) batteries like NiMH or Li-ion can be recharged hundreds of times, making them more economical and environmentally friendly.

• Why does the same battery size come in different capacities?

  Different chemistries have different energy densities. A Li-ion 18650 holds 2500-3500 mAh while a NiMH AA holds 1800-2800 mAh. Even within the same chemistry, capacity varies by manufacturer and quality grade.

• Can I replace AA batteries with rechargeable versions?

  Usually yes, but note that NiMH AA cells are 1.2V vs 1.5V alkaline. Most devices work fine, but some may detect the lower voltage as low battery. Lithium AA cells maintain 1.5V if needed.

• Which battery size has the best energy density?

  Among common consumer formats, lithium-ion 21700 cells are often the strongest balance of capacity and size, which is why they are widely used in power tools and EV modules. Exact results still depend on chemistry, manufacturer, and pack design.

• How do I choose between parallel and series for battery packs?

  Series connections increase voltage (e.g., 2 × 3.7V = 7.4V). Parallel connections increase capacity (e.g., 2 × 3000mAh = 6000mAh). Many packs combine both: a 3S2P pack has 3 series groups of 2 parallel cells.