What are tensor data types (dtypes) in PyTorch and why do they matter?

Every tensor has a dtype that determines the numeric type and precision of its elements. Choosing the right dtype affects memory usage, computation speed, and numeric precision — a critical consideration when training on GPUs.

import torch

# Creating tensors with specific dtypes
x = torch.tensor([1.0, 2.0], dtype=torch.float32)
y = torch.tensor([1, 2, 3], dtype=torch.long)     # class labels
m = torch.tensor([True, False, True], dtype=torch.bool)

# Casting between dtypes
print(x.dtype)             # torch.float32
x64 = x.double()           # → float64
x16 = x.half()             # → float16
xi  = x.to(torch.int32)   # → int32

# Default dtype (float32 for floats, int64 for ints)
print(torch.tensor([1.0]).dtype)   # torch.float32
print(torch.tensor([1]).dtype)     # torch.int64

# Change global default
torch.set_default_dtype(torch.float64)  # rarely needed

# Why dtype matters for loss computation:
# CrossEntropyLoss expects:
#   input:  float32  (logits)
#   target: int64    (class indices)
loss_fn = torch.nn.CrossEntropyLoss()
logits = torch.randn(4, 10)                 # float32
targets = torch.randint(0, 10, (4,))        # int64
loss = loss_fn(logits, targets)             # works!
# targets_wrong = targets.float()           # would error!

Most common dtype errors: passing float64 weights into a model expecting float32, or passing float targets to a loss function expecting long (e.g. CrossEntropyLoss).

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.