JSON Number Handling — int vs float64 in Go

JSON Number Types in Go

JSON has a single number type, but Go distinguishes between int, int64, float64, and more. Choosing the right type matters for correctness and precision.

Default Behavior

When unmarshaling into interface{}, all JSON numbers become float64:

var data map[string]interface{}
json.Unmarshal([]byte(`{"id": 9007199254740993}`), &data)
// data["id"] is float64, loses precision!

float64 can only represent integers up to 2^53 precisely. Larger IDs (like Twitter snowflake IDs) lose precision.

Using json.Number

decoder := json.NewDecoder(reader)
decoder.UseNumber()

With UseNumber(), JSON numbers unmarshal as json.Number (a string type), preserving the original text:

n := data["id"].(json.Number)
id, _ := n.Int64()  // precise

Type Selection Guide

JSON value	Go type	When to use
42	int	Small integers, counts, IDs
9007199254740993	int64	Large integers, timestamps
3.14	float64	Decimal numbers, coordinates
"42"	int with string tag	Numbers sent as strings
any number	json.Number	When you need the raw text

Struct Field Types

type Transaction struct {
    ID     int64   `json:"id"`
    Amount float64 `json:"amount"`
    Count  int     `json:"count"`
}

The converter analyzes the sample JSON: numbers with decimals become float64, whole numbers become int. Very large numbers may trigger int64.

Numbers as Strings

Some APIs quote numbers: "amount": "19.99". Use the string tag option:

Amount float64 `json:"amount,string"`

Precision-Critical Applications

For financial data, avoid float64 entirely. Parse the string representation and use an arbitrary-precision library or store cents as int64.