Question 842021
A diagram in a wikipedia article suggests that for dead wood, the portion of carbon as C-14 is 3.6%.  Your piece of wood would have an estimated {{{(0.036)(3)=0.108}}} milligrams of C-14 now.


DECAY EQUATION
{{{A=Ie^(-kt)}}}, I initial amount, t years, A amount at t years, k a constant, e Euler Number
Half-Life A=I/2, so equation becomes {{{I/2=Ie^(-kt)}}}
{{{(1/2)=e^(-kt)}}} and we also expect t=5730 for this half-life,
{{{(1/2)=e^(-k*5730)}}}
{{{ln(1/2)=-k*5730*ln(e)}}}
{{{-k*5730*1=ln(1/2)}}}
{{{k=(-ln(1/2))/5730}}}
{{{highlight_green(k=0.000121)}}}
-
The decay equation may be {{{highlight(A=Ie^(-0.000121*t))}}}


Your question specifies a calculable 0.108 milligrams of Carbon 14 at present now, and to find how much will be present for t=1000 years.
-
I=0.108 and t=1000;
Find A.
{{{highlight(A=(0.108)e^(-0.000121*1000))}}}