# Is \`BigFloat\` loss of precision intended?
**URL:** https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728
**Category:** Internals & Design
**Tags:** precision, bigfloat
**Created:** [May 24, 2021, 3:51pm UTC](https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728 "2021-05-24T15:51:56Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@MarcMush](https://discourse.julialang.org/u/MarcMush)
#### Post date: [May 24, 2021, 3:51pm UTC](https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728/1 "2021-05-24T15:51:56Z")
when doing any operation on a `BigFloat`, the precision of the result always reverts back to the default one, instead of getting it from the operands.
Is this behavior intended and if yes, why ?
```julia
julia> a=BigFloat("1.1", precision=512);
julia> a.prec
512
julia> (a+a).prec
256
julia> (0+a).prec
256
```
---
### Author:  [@gustaphe](https://discourse.julialang.org/u/gustaphe)
#### Post date: [May 24, 2021, 4:56pm UTC](https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728/2 "2021-05-24T16:56:10Z")
I don’t know, and I don’t know why it’s like this, but maybe
```julia
setprecision(512) do
x = big"1.1"
x+x
end
```
solves your problem?
---
### Author:  [@MarcMush](https://discourse.julialang.org/u/MarcMush)
#### Post date: [May 24, 2021, 5:06pm UTC](https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728/3 "2021-05-24T17:06:25Z")
I know about `setprecision`, but I was wondering the point of the `precision` keyword if it doesn’t propagate
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### Author:  [@gustaphe](https://discourse.julialang.org/u/gustaphe)
#### Post date: [May 24, 2021, 6:50pm UTC](https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728/4 "2021-05-24T18:50:20Z")
Hm. `base/mpfr.jl` defines these functions, and it’s an immediate `ccall`. I’m not sure there’s much one can do without changing it to `BigFloat{512}` etc. But I don’t know a lot about it.
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### Author:  [@thofma](https://discourse.julialang.org/u/thofma)
#### Post date: [May 24, 2021, 6:52pm UTC](https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728/5 "2021-05-24T18:52:23Z")
No, it is not an immediate ccall. It first allocates the result using `BigFloat()`, which uses the global precision and not the precision of the input (which it could).
PS.: So yes, this is intended. The precision of the result is always the global precision. This is the same behavior as MPFR itself.
---
### Author:  [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)
#### Post date: [May 24, 2021, 7:03pm UTC](https://discourse.julialang.org/t/is-bigfloat-loss-of-precision-intended/61728/6 "2021-05-24T19:03:06Z")
You must use `setprecision(BigFloat, nbits)` for precision to work properly (at nbits). The precision keyword is there because over time we have been considering how to rework the whole interface.