How to compute the JSON length?

Use measureLength() to compute the number of characters that will be printed by printTo().

Use measurePrettyLength() to compute the number of characters that will be printed by prettyPrintTo().

None of these methods includes the zero-terminator; so, if you need to allocate a buffer, don’t forget to add 1 to the size.

StaticJsonBuffer<200> jsonBuffer;
JsonObject& root = jsonBuffer.createObject();
root["hello"] = world;

size_t len = root.measureLength(); // returns 17

size_t size = len+1;
char json[size];
root.printTo(json, size); // writes {"hello":"world"}