32GB = approximately 28GB-30GB
64GB = approximately 58GB-60GB
128GB = approximately 115GB-120GB
256GB = approximately 230GB-240GB
This is calculation difference between manufacturer and your PC, please Google 'Memory card capacity' to get more information.
Your PC says 1GB = 1.073741824 Billion bytes where Market Defines 1GB = 1.00 Billion bytes.

Compatibility:

For microSDHC (32GB) card: Compatible with microSDHC and microSDXC host devices
For microSDXC (64GB, 128GB, 256GB) card: Compatible with microSDXC host devices.
Dimensions (L x W x H): 0.04" x 0.59" x 0.43"
Sequential Read Performance: Up to 100MB/s
Sequential Write Performance: Up to 40MB/s

DISCLOSURES

* 1GB = 1,000,000,000 bytes. Actual user storage less.
1. Actual hours of video saved less. Up to 20,000 hours for 256GB; 10,000 hours for 128GB; 5,000 hours for 64GB; and 2,500 hours for 32GB. Full HD video only; total hours less for 4K UHD video.
2. Card only. See product packaging and www.sandisk.com/proof for more information and limitations.
3. Compatible device required. Full HD (1920x1080) and/or 4K UHD (3840x2160) video support may vary based on host device, file attributes, usage conditions and other factors. See www.sandisk.com/HD
4. UHS Speed Class 3 (U3) and UHS Speed Class 1 (U1) designate performance options designed to support real-time video recording with UHS-enabled host devices. Video Speed Class 30 (V30), sustained video capture rate of 30MB/s, designates a performance option designed to support real-time video recording with UHS-enabled host devices. See www.sdcard.org/consumers/speed.
5. Up to 100MB/s read speed, up to 40MB/s write speed. Based on internal testing; performance may be lower depending on host device, usage conditions and other factors. 1MB=1,000,000 bytes.

Package Contents:

SanDisk microSD Card with Retail Package*1